(CNN) -- As it turns 20 on Thursday, "The Simpsons' " greatest enemy may be itself.

For many fans -- particularly hard-core followers in the mold of the show's sneering Comic Book Guy -- the glory days are long past. Some refuse to watch anymore; others admit they still find it funny, but they're disappointed the show didn't bow out at the top of its game.

Jacob Burch, an administrator of the "Simpsons" site NoHomers.net, is one of those fans.

The characters have gotten flat, says Burch -- who, at 23, has practically been watching the show his whole life -- and it's more likely to go for cheap laughs nowadays.

"It seems less cohesive, more about trying to get the jokes in there, instead of make a story and let the jokes come off of that," he says, adding, "I just think there's only so much you can do [with the characters]."

On the site, Burch now focuses on the show's history, letting the more passionate fans moderate the chats about current episodes.

John Ortved, author of a new oral history of the show, "The Simpsons: An Uncensored, Unauthorized History" (Faber and Faber), agrees. "It's clearly not as good," he says. "I think the only people who think it's good -- or as good as it was -- are [producer] Al Jean and [co-creator] Matt Groening."

Ortved makes the case that despite new writers and characters, the show has fallen out of touch and gotten more craven, playing to guest stars and cross-promoting other Fox shows. "What's been described to me is ... Al Jean just doesn't get it," he says. "The young, hip writers are either getting their jokes annihilated by Al Jean and his sort of yes men, or they've stopped writing them because they know they're going to get rewritten anyway."

While not directly addressing the complaints, Groening and Jean have said "The Simpsons," whose 20th anniversary special is scheduled for January, isn't going anywhere. In February, Groening told CNN that he'd "be surprised if we close up anytime soon. ... The show's still fun to do."

And Jean, who has acknowledged the criticism in interviews, has said he believes the show is still potent. (Neither was available for interviews for this piece.)

Most observers agree the show has declined from its heyday, defined by fans as roughly lasting from seasons three through eight. But then, those episodes set an extremely high bar. Sidebar: The "Simpsons" comedy tree

In the 1990s, "The Simpsons" was one of the most inventive shows ever broadcast, taking on high and low culture with equal abandon, becoming engrained within the culture at large. It was revolutionary; at the very least, it helped make Fox a big-league player.

Today, with its 442 episodes airing all over the world, it's "like the new Disney ... it's iconic," says Chuck Coletta, a pop culture instructor at Bowling Green State University. (Indeed, it's iconic enough to have a ride at Disney competitor Universal Studios Hollywood.)

Colleagues and some fans stand behind Groening and Jean. Seth MacFarlane, whose "Family Guy" has become, to some, the darling "The Simpsons" once was, told Ortved that "it is still funnier than any live-action show that's on television right now. ... 'The Simpsons' has sustained better than 'South Park.' "

And John O'Leary, a Villanova University pop culture professor who has taught "The Simpsons" in his courses, says, "I still enjoy the show. ... I still turn it on and laugh."

Entertainment is full of stories about "jumping the shark," as the plunge into decline has come to be known. Rock 'n' roll fans have mocked the last 25-plus years of Rolling Stones albums; Orson Welles' career is seen as a defining case (though whether the decline started after "Citizen Kane," "Touch of Evil" or "F for Fake" is constantly debated).

In television -- given the competing components of artistic creativity, ratings success and profitability -- picking the right time to say "enough" is a challenge.

Some shows handle the transition gracefully. "The Sopranos" was lauded for going out strongly (though, critics gripe, it wasn't as good as it was in the early days); so were "The Bob Newhart Show" and "The Mary Tyler Moore Show." Some of the most-praised British shows -- "Fawlty Towers" and "The Office" -- had deliberately short runs and never declined.

But such timing is rare; if they're successful with viewers, TV shows will tend to hang around too long -- and then everything falters at once. By the time it was canceled in 1975, "Gunsmoke" -- which "The Simpsons" recently surpassed in longevity -- was a creaky facsimile of the vibrant, thoughtful Western that dominated television in the late '50s, O'Leary says. "It became about the guest stars," he says. (Some have made the same claim about "The Simpsons.")

"The Simpsons" does have a deep well to draw from, maintains Coletta. In Springfield, nobody ages and there's always someone else -- Mrs. Krabappel, Groundskeeper Willy -- or a new trend to do a story about. Other shows, he observes, run out of steam because actors get older or leave. (In recent years, of course, some live-action shows such as "Law & Order" and "ER" have benefited from cast turnover.)

And as long as a show is attracting viewers, it tends to stay on the air -- especially nowadays when the overall audience for broadcast TV has plunged. "The Simpsons" hasn't been immune to audience erosion, but it still beats its Sunday night competition and does well with the desirable adults, the 18-49 demographic.

It also remains a gold mine. Ortved estimates the show has earned $3 billion over its run, thanks to worldwide syndication and its broad-based empire of merchandising. Maintaining the show is key to the riches, says Michael Stone, CEO and president of the licensing and marketing firm the Beanstalk Group.

"As a vehicle [for licensing], the show is worth having," he says. "Without the show, I think the property is in serious decline." Even "Star Wars" has faded without the films in theaters, he points out.

Hollywood economics also argue for keeping the show on the air. As producer Bill Lawrence, who agreed to bring "Scrubs" back this season, told The New York Times, "In this economic landscape, if you have the chance to continue a project, you don't just say: 'No big deal. I'll go work somewhere else.' "

O'Leary, who studied at UCLA, say his friends in the business are struggling for jobs; a sure paycheck such as "The Simpsons" is attractive, regardless of quality concerns.

Moreover, "The Simpsons" continues to attract new audiences. The show airs in more than 90 countries and still appeals broadly: Youngsters appreciate the bright colors and manic pace; older viewers get jokes about "The Jazz Singer" and media consolidation. Many of O'Leary's and Coletta's students weren't even alive when the show went on the air -- and, even as adults, don't get all the jokes.

"Some of the guest stars on the early episodes -- it's almost like watching an episode of Bugs Bunny when Greta Garbo shows up," Coletta says, noting that many of his students would fail to understand the significance of Elizabeth Taylor's appearance in the fourth season.

He sees the desire to knock "The Simpsons" down (while hoping for a comeback) as "human nature" -- but there's no denying the show's impact.

"You see it with [other shows] -- 'Lost' is the greatest show in the world, then 'Lost' stinks now, then 'Lost' has made a comeback. We just do this over and over and over again," he says. "But I think the big thing about 'The Simpsons' is that, I'm teaching a class that's filled with freshmen now, and they don't know a world without 'The Simpsons.' It's part of life."

Even for skeptics such as Burch. Noting that it's still capable of brilliance, he says he wishes the show well -- even if he can't hide his disappointment.

"It's still, I'm sure, better than the average TV show," he says. "If it's still profitable and the people making it are still enjoying themselves, I don't see why [it can't continue], because every now and then there will be that one episode that has a new mind or a great idea or a great new character."

And Ortved? Even with all he learned about the show -- its backbiting, its disappointments, its becoming the thing it once mocked -- he can't help but admire it. Even today.

"I still love the show," he says.