This is the third part of a three-part series. At least, I'll pretend I planned it as a three-part series. Last week, I wrote about how the American League is still the better league, but that the success of the Chicago Cubs may force the rest of the NL to up its game a bit, similar to what happened in the AL in the mid-2000s as the rest of the league tried to catch up with the New York Yankees and Boston Red Sox. On Monday, I looked at how the other 29 teams can beat the Cubs in 2017. That article ignored what's most likely to happen to the Cubs in 2017: They simply won't be as good.

How do we know this? Since 2000, 15 teams besides the 2016 Cubs won at least 100 games. Only one of those 15 teams improved the following season. The 2001 Oakland A's won 102 games, and then won 103 in 2002. Two of the 15 teams matched their win total: The 2002-03 Atlanta Braves and 2003-04 Yankees both won 101 games in back-to-back years. The average decline was minus-7.9 wins. So based on simple historic precedent, the Cubs should be pretty good again, just not quite as good. Years ago, Bill James coined this the Plexiglas Principle.

This idea of regression isn't just limited to 100-win teams. Seven teams won 90 games in 2015; all except the Cubs won fewer games in 2016. Six teams won 90 games in 2014; only one improved in 2015. Ten teams won 90 games in 2013; only one improved the following season, and the collective decline averaged 11 fewer wins. If anything, it's even harder now to win 100 in consecutive seasons than it was in the early part of the century.

Are the Cubs similar to our 15 previous 100-win teams? Here's a chart showing all 16 teams with their win total, that season's Pythagorean record (estimated wins based on runs scored and allowed), the following season's win total, the average age (weighted for playing time) for position players and pitchers from Baseball-Reference.com, and the gain or loss in runs scored and runs allowed the following season:

100-Game Winners A look at how recent 100-game winners fared the following season: Year Team Wins Pythag Next Gain Bat Pitch Runs RA 2016 Cubs 103 107 ?? ?? 27.4 29.9 ?? ?? 2015 Cardinals 100 96 86 -14 28.4 28.2 132 187 2011 Phillies 102 103 81 -21 31.5 29.2 -29 151 2009 Yankees 103 95 95 -8 30.4 29.3 -56 -60 2008 Angels 100 88 97 -3 28.9 27.6 118 64 2005 Cardinals 100 98 83 -17 30.8 29.7 -24 128 2004 Cardinals 105 100 100 -5 30.6 30.1 -50 -25 2004 Yankees 101 89 95 -6 32.3 28.4 -11 -19 2003 Yankees 101 96 101 0 30.5 27.4 20 108 2003 Braves 101 96 96 -5 30.3 31.1 -104 -72 2003 Giants 100 93 91 -9 32.1 28.6 95 132 2002 Yankees 103 99 101 -2 28.6 33.1 -20 19 2002 A's 103 96 96 -7 28.6 27.4 -32 -11 2002 Braves 101 96 101 0 30.6 31.1 199 175 2001 Mariners 116 109 93 -23 31.4 30.8 -113 72 2001 A's 102 104 103 1 27.4 28 -84 9

Ten of the 15 teams scored fewer runs the following season, although the average gain overall was plus-2.7 runs (the 2002-03 Braves improved 199 runs on offense). Only five of the 15 teams allowed fewer runs the following season. One of those was negligible (minus-11) and the 2009-10 Yankees benefited from a leaguewide decrease in offense. The average decline was 57.2 more runs allowed in the following season.

That's the most likely scenario for why the Cubs don't win 100 again: They allow more runs.

Start with Kyle Hendricks. As ESPN Insider Jeff Sullivan wrote recently, Hendricks likely had his career season, in part because few pitchers can repeat a 2.13 ERA. Sure, Sandy Koufax did it four seasons in a row and Clayton Kershaw has done it four seasons, but that's kind of the point: Hendricks probably isn't Koufax or Kershaw. Jeff wrote:

This has all gotten terribly complicated. And it makes me additionally feel a little bad, because I like Kyle Hendricks, and I think he's going to remain a quality starter. It's possible I haven't gotten that point across, in trying to tear down his future ERAs from his 2.13 mark in 2016. The point, in short, is this: In 2016, Kyle Hendricks got the absolute most that he could out of his skill set. He also got the most that he could out of the skill set of his collective defenders. Just from Hendricks' own defenders getting worse, he's going to allow some more runs. And Hendricks himself presumably overachieved, for any number of minor reasons that add up over time.

Jon Lester's ERA dropped from 3.34 to 2.44. He had nearly identical strikeout, walk and ground ball rates, but allowed 29 fewer hits in just three fewer innings. He wasn't the exact same pitcher as in 2015 -- he threw his fastball more and his cutter less often -- but as with Hendricks, it seems Lester received substantial benefit from the Cubs' great defense. Of course, that defense could be just as good again in 2017, but that circles back to why teams win 100 games. A lot of things go their way, including defense.

The Cubs also had remarkably good health. Other than Kyle Schwarber's season-long knee injury, the team suffered no major injuries. Dexter Fowler was out for a few weeks and Jorge Soler had a DL stint, but Anthony Rizzo, Kris Bryant, Addison Russell, Ben Zobrist, Javier Baez and Jason Heyward basically played every day. One reason they were healthy is that all except Zobrist were 27 or younger. If you go back to the chart, only the 2001 A's had a position player core as young as the Cubs'. That's an obvious reason for optimism. Starting in 2000, that A's team won 91, 102, 103, 96, 91, 88 and 93 games -- without the financial resources the Cubs will have to reload as necessary.

The starting rotation was even healthier. All five starters made at least 29 starts, as Joe Maddon had to use only 10 starts from outside his top five guys. Two of those were spot starts the final few days of the season, and five went to Mike Montgomery when Maddon went to a six-man rotation for a spell in late August and early September. In the wild-card era since 1996, only 15 teams had five starters make at least 29 starts. No team did it in consecutive seasons. More than likely, the Cubs will have to dig much deeper into their starting pitching depth in 2017.

Finally, the rest of the NL should be tougher. The Cubs went 15-4 against the Cincinnati Reds, outscoring them by an average of 3.9 runs per game. They went 14-4 against the Pittsburgh Pirates, a team that didn't have one starter get to even 22 starts. In a league with a big split between the good teams and the bad teams, the Cubs benefited from a weak division. They played just 56 games against teams that finished over .500, going 31-25, and had just 31 games against playoff teams (the Los Angeles Dodgers, by contrast, had 51 games against playoff teams). The Reds, Milwaukee Brewers, Pirates and St. Louis Cardinals could all be better in 2017.

Now, the Cubs could certainly win 100 again. There's good reason to think the offense will score more runs: a healthy Schwarber, a full season behind the plate from Willson Contreras, improvement from Russell and Baez, some bounce-back from Heyward. It's possible Bryant hasn't maxed out yet. History, however, suggests that winning 100 again will be a tall order.