The San Francisco 49ers are, in some circles, the trendiest Super Bowl pick going. They return 21 of 22 starters from last year, when they went 13-3 and came within a questionable call of making it to the Super Bowl. They even went out this offseason and upgraded their weakest position, wide receiver, by bringing in a Hall of Famer and the guy who made the biggest play in that Super Bowl. Their style of play — built around dominating field position and winning the turnover battle — is associated with the simplest narratives of classic winning football teams. And they’re led by a head coach who might have been the best in football during his first season at this level. The case for the 49ers throwing up a second consecutive dominant season is simple enough, and it’s one that the gambling public is buying into.

On the other hand, I personally think the Niners are going to win nine games or fewer. I see a team whose performance is unsustainable in a number of ways, one that will struggle mightily to re-create all the advantages they had in 2011. I think of it like a credit score: There’s no one individual item that guarantees a Niners collapse, but instead a variety of extraordinary factors driving last year’s success erode their current expectations from multiple angles. San Francisco simply has too much to overcome in 2012 to repeat their 13-win season. And once you’ve seen the evidence in context, I suspect that you’ll agree with me.

It makes sense to start with the simplest metric of all: Wins. The Niners improved by seven games last season, as their disappointing 6-10 season from 2010 gave way to a 13-3 juggernaut last season. That’s a seven-win improvement, and as I noted in the over-unders column last week, it’s been an unsustainable one. Since the advent of the 16-game season in 1978, 10 other teams have improved by seven wins in a given season. During the subsequent year, those teams won an average of 4.7 fewer games. Ten teams is a pretty small sample, though, so let’s expand it. Since 1984, 31 teams have improved by a figure between six and eight wins in a given season. Of those teams, 25 declined after their big leap forward. Collectively, those 31 teams won an average of 3.2 fewer games the following season.

That decline doesn’t happen because of the math, of course; it happens because there are simply too many things that have to go right for a team to repeatedly put up 13-3 seasons. Independent of their track record in previous years, the average 13-3 team has won an average of just 9.3 games the following year, with just three of the 32 examples maintaining their 13-win record from the previous year. That leads to the bigger question: Which aspects of San Francisco’s performance from last year are unsustainable?

Well, let’s remember what the Niners were good at and start from there. More than anywhere else, Jim Harbaugh’s team established themselves as a squadron that consistently dominated the turnover battle. San Francisco only lost that turnover battle in three games last year, and they lost two of them. The Niners finished the regular season with a turnover differential of plus-28; that was the best in football, with only the plus-24 figure of the Packers anywhere close.

That sort of turnover differential just isn’t going to happen again, unfortunately. There have been 21 other teams since 1978 who produced a turnover margin of plus-20 or more, and as you might suspect I’m about to say, they weren’t able to keep it up. The following year, their average turnover differential was just over three. Not per game. Over the whole season. On average, they shifted by more than 20 turnovers over the course of the campaign. That decline was joined by a decrease in wins: Those teams won an average of 2.5 fewer games the following season.

There are reasons to think that both the Niners offense and defense will cede their turnover advantage in 2012. Start with the Alex Smith–led offense. Last year, they had the fewest interceptions per drive and the sixth-fewest fumbles per drive. That combination meant that they only turned over the ball once every 18.5 drives, which was the best rate in football.

While the Niners will likely fumble a couple more times in 2012, the more predicative (and meaningful) figure here is Alex Smith’s interception rate. Smith — a quarterback who had thrown interceptions on 3.5 percent of his passes before 2011 — threw picks on just 1.1 percent of his attempts last season, a figure which led the league and rated as the fifth-best figure in the history of the National Football League. That’s from the same guy who was inspiring “We Want Carr” chants 10 months earlier.

If it’s safe to bet on any stat correcting itself from year to year in football, it’s interception rate. Before last season, I mentioned the fluky-low interception rates of guys like Tom Brady (0.8 percent), Josh Freeman (1.3 percent), Ben Roethlisberger (1.3 percent), and Matt Cassel (1.6 percent). Each of those players saw his INT rate double during the 2011 campaign. In fact, if you go back and look at every player who has produced an interception rate of 1.5 percent or less over 300-plus attempts (virtually all of which occurred after 1990), they as a group threw interceptions twice as frequently during the subsequent season.

While part of Smith’s low interception rate is undoubtedly due to pure luck, let’s be realistic about putting it in context. If you watched Niners games last year, you know that San Francisco really built their entire offense around keeping Smith from making bad throws and producing turnovers. If that meant checking down on third-and-whatever and punting, doing so wasn’t a problem. That hyperconservative offense undoubtedly helped keep Smith’s interception rate below that of a quarterback in a more typical offense.

The problem with expecting that low interception rate to continue is that it won’t make sense for the Niners to build an offense like last year’s. They’ve brought in Mario Manningham and Randy Moss, players who are expected to stretch opposing defenses vertically. They’re supposed to upgrade that weak spot of the San Francisco offense, but if Smith is still going to be the checkdown machine that he was a year ago, how can they really have an impact? It’s a catch-22. If the Niners stay with their old scheme, Moss and Manningham will mostly be useless. And if they expand their offense to take more shots downfield to their new weapons, Smith is going to take more risks and see his interception rate shoot back up to where it once was. You can’t expect Smith to simultaneously integrate Moss and Manningham into the offense and keep his interception rate low. He just doesn’t have that sort of ability.

An increase in turnovers will also hurt San Francisco’s generous field position from last season. Last year, the 49ers defense took over with an average of 76.1 yards of real estate behind them, the best figure in football. And because they were so good on defense, the Niners gave the ball back to their offense with an average of just 66.6 yards to go for a touchdown, which was also the best starting field position in football for an offense. Even if the Niners continue to place a premium on field position, it’s extremely unlikely that they’ll lead the league on both sides of the ball again. Football Outsiders has drive statistics going back through 1997, and only one team over that time frame — the 2006 Ravens — led the league in average starting field position on either side of the ball. During the following season, their average starting offensive field position was 23rd, and their average field position on defense was … last in the league. Yikes.

The Niners were certainly takeaway-happy to an unsustainable extent in 2011. The drive stats there have them first in the league in percentage of drives ending in fumbles and third in drives ending in interceptions, producing a takeaway rate matched only by Green Bay. That’s also not going to stick around in 2012. For one, the Niners forced 20 fumbles on defense and recovered a league-high 15 of them. Forcing fumbles is a skill, but recovering them is mostly a random act. Consider that the 2010 Niners forced 12 fumbles on defense and recovered six of them, while the 2009 unit forced 21 fumbles and recovered 15. And as for the interceptions, well … I’ll spare you the numbers, but don’t put me on the list of people who think that Carlos Rogers is suddenly going to pick off a half-dozen passes per year from now on. Safety Dashon Goldson is a legitimate ball hawk and playmaker, but even he is unlikely to produce six picks in consecutive seasons. In all, San Francisco can expect to decline in every meaningful aspect of the turnover game in 2012.

One of the other reasons that defense was so effective: health. Last year, San Francisco’s 11 starters on defense combined to miss a total of just eight games due to injury. Three of those games belonged to Patrick Willis, who probably could have come back earlier from a pulled hamstring if the Niners hadn’t been playing out a string of meaningless games during his absence. And if that seems impressive, consider that the 2010 Niners defense had 11 starters who missed a total of just two games all year. If you want to argue that the Niners just have a group of healthy, hungry players on defense, remember that they swapped out five starters last offseason. And on offense, players with injury histories, like Smith, Frank Gore, and Michael Crabtree, made it through the season relatively unscathed. Outside of Josh Morgan, who went down for the season in Week 5, the other 10 starters on offense missed a total of two games. The Niners aren’t due to suffer a rash of injuries in 2012 — that’s the gambler’s fallacy — but the randomness of injuries suggests that they’re extremely unlikely to be as healthy this upcoming season. We all saw how an injury stack affected their play during the NFC championship game, when Kyle Williams was forced into a key role and fumbled two punts. The effects of injuries are usually more subtle, but they should cause the Niners more problems in 2012.

The Niners were also somewhat lucky in terms of the season-long stats that I often discuss in this space. It actually wasn’t really the case with their Pythagorean projection, since their point differential was that of a 12.3-win team. On the other hand, while they lost a heartbreaker of a close game to the Cowboys in Week 2, the Niners were an impressive 6-2 in games decided by one touchdown or less. The 20 teams who did that before them went 80-83 in those same games the following year. I adore Jim Harbaugh as a tactician and game manager on the sidelines, but even he’s not good enough to win 75 percent of his close games.

San Francisco also had a friendly slate of opponents last season. Football Outsiders estimates that the Niners faced the easiest schedule in football last season, while Pro-Football-Reference.com has the Niners with the sixth-easiest set of opponents. That owes mightily to the NFC West, a division that will still likely be subpar in 2012, but the rest of the slate will also be tougher. San Francisco will face the two toughest divisions in football this season, with eight games against the NFC North and AFC East. That includes trips to Green Bay and New England. They’ll play the Giants as one of their two placement games for the second year in a row, but the schedule swaps out the Buccaneers for the Saints this season. If the Rams and Seahawks deliver on their spurts of competency from the past two seasons, the Niners could face one of the toughest schedules in football. It will almost surely not be the easiest slate in football for a second consecutive year.

From what I can tell, the only real argument against the fate seemingly awaiting the Niners is the presence of Jim Harbaugh. Perhaps, Niners fans seem to hope, the roster was really a 13-win team all along, just one held back by the bumbling misanthropy of Mike Singletary. I can understand where they’re coming from. I’ve said on the record that I believe the Singletary-to-Harbaugh swap to be the biggest head coaching improvement an NFL team has ever made. Perhaps those Niners really were much better than they played.

But even if Harbaugh’s every bit as good as I think he is, it wouldn’t be a surprise to see his team slip in 2012. I put Harbaugh in a class with two other head coaches tactically, and they each had a similar fall. Bill Belichick followed a 5-11 debut season with the Patriots by leading New England to a stunning 11-win season, one that ended with a Super Bowl win. The year after, Belichick’s team went through a disappointing 9-7 campaign that saw them miss the playoffs. Sean Payton’s Saints, meanwhile, pulled off their stunning 10-6 NFC South win during Payton’s debut season with the team, one that ended in the NFC championship game. After that season, they retreated to 7-9 and spent two years in the wild before winning the Super Bowl in 2009. It’s not like Payton was a terrible coach for those two years, either. This year, coaches will adjust a bit to Harbaugh. His bag of tricks — the sneaky offsides move, the unexpected onside kicks — won’t work quite as well. The 2012 49ers can simultaneously have a great coach and still be disappointing.

The 2012 49ers can also simultaneously be disappointing and have a productive season. There’s no reason to think that the Niners will totally collapse and finish with a 4-12 record; there’s too much talent and too great a coach for that to happen. In all likelihood, San Francisco is primed for a season like those 2007 Saints or 2002 Patriots, an 8-8 campaign that reveals some fixable flaws in their constitution. That could be enough to win the NFC West and get into the playoffs, but 49ers fans dreaming of a Super Bowl run might want to dream about life with a real quarterback instead. As constructed, the Niners will find it extremely difficult to be an elite team year-in and year-out. When everything goes right for a team like San Francisco, you get 2011. When some of those things go wrong, you get 2012.