Sitcoms are fairly straightforward in terms of conception. A central premise is established in the pilot episode, one that that usually has an obvious end goal. The episodes essentially riff on this premise, following a meandering progress to that end goal by prodding the characters towards it. The best sitcoms end when the premise runs its course, but the financing of such shows usually works against good structural plotting. This is what's happened to The Big Bang Theory, though from a network perspective, it's still profitable and therefore who cares?
To explain what I mean, I'll take a look at some other successful sitcoms. There's the ur-sitcom, I Love Lucy, which ran for six seasons, and was funny to the end, though it might not have been if Lucy and Ricky hadn't moved to Connecticut, opening the way for situations impossible in New York City, like a lawn mower run amok on prize flower beds. The series ends abruptly - there essentially is no finale, but that's because the show didn't so much end as morph into The Lucy-Desi Comedy Hour, hour-long specials that aired over three years. It was a forum for Lucy to interact with movie stars, while Ricky, Fred, and Ethel, although still part of the series, participated far less. The second incarnation, though successful at first from a financial perspective, was ultimately a lackluster shadow of the original sitcom. The reasons for the change were myriad, but a primary reason was that Desi Arnaz was having health problems (quite obvious in many of his scenes, especially the later ones). I Love Lucy remains an iconic show, still laugh-out-loud funny almost sixty years after its final episode first aired. Its reputation was cemented during syndication and it's important to note that the The Lucy-Desi Comedy Hour was edited into half hour segments, shedding much of the bulk. I Love Lucy transformed television comedy and was one of the most-watched shows in American history. It's exceptional by nature. But its continuation was lousy because the central premise had fallen apart. With Ricky, Fred, and Ethel sidelined, the show faltered and began to feel repetitive and dull.
The Nanny, Fran Drescher's take-off of The Sound of Music via Flushing, Queens, achieved a great deal of popularity while it was airing and even more so in its dubbed versions abroad in countries like France and Italy. The show ran for six seasons, what seems to be the magic number for sitcoms. The premise - streetwise Jewish-American nanny falls in love with her millionaire British boss - tended towards the obvious conclusion - streetwise Jewish-American nanny marries millionaire British boss. The show's pacing was impeccable. At the end of the fourth season, when the tension began to wear thin, it was clear that the relationship between Fran and Maxwell was heating up. They were married at the end of the fifth and had twins at the end of the sixth, moving to California and leaving behind the mansion set that had been a constant throughout the show. Accordingly, the show never had a chance to falter or wander off into too many episodes that turn out to be dreams or fantasies. Although Fran Drescher has since said that she was pressured into wrapping up The Nanny before she thought it necessary, frankly the show has survived as well as it has because it didn't wear out its welcome.
That '70s Show, on the other hand, which premiered as The Nanny was wrapping up, stayed strong until its seventh season. It ran for eight. The show's pacing was marred by a (frankly rather psychedelic) stretching of time in order to accommodate additional seasons while keeping the characters in the 1970s. In the seventh season, the premise - group of high school friends centered around Eric Forman hang out, date each other, and get high in his parents' basement - had begun to collapse. The characters had graduated from high school and the plots were increasingly scattered and unconnected with each other. The show could have ended on a high note in season six, and it could have ended with a certain sentimentality and end-of-the-road feeling in season seven. Instead, it continued for an eighth season, an appallingly bad one. The anchor of the plot was Eric because all of the characters in one way or another relate to each other through him. Once Topher Grace left, the show completely foundered. Eric's father became a mouthpiece for the audience, crankily wondering why the show's remaining cast members insisted on continuing to hang out in his basement. What really shredded the show beyond redemption was the addition of a love interest for Donna, Randy, played by Josh Myers and perhaps one of the most irritating characters of all time.
Friends ran for even longer, reaching a full ten seasons. The show had one strength in the extreme openness of its premise - six friends live their intertwined lives as young twenty- and thirty-somethings in New York. The humor was character-based and the plots, though heavily engaged in the romantic entanglements of the characters both with each other and with guest stars, also focused on career and parenting problems, which gave the show a wider scope than, for example, The Big Bang Theory, which hyper-focuses on the romantic. It also helped that each of the characters had unique, developed relationships with all of the other characters, so it was unnecessary to lean on romantic couples as basic units to drive forward plots, even as the basic end goal for the show was to finally get Rachel and Ross back together. Friends certainly got pretty tired by the end and probably should have ended after the ninth season, but it's one of the few such shows to survive so many seasons without imploding. It descended into sentimentality at the end, as had The Nanny, though both shows had loyal followings involved enough in the characters' fates that it mattered less when those final episodes first aired.
The most painful casualty of having too many seasons, as far as I'm concerned, is Will and Grace. It ran for eight seasons and truly ran itself into the ground. The premise of the show - straight woman and gay man are best friends in a marriage-like relationship - was aiming at, like most sitcoms about single people, settling its characters romantically. When Grace married Leo in the fifth season, it should have signaled the end of the line, since from there, the end goal established at the beginning had been half fulfilled. From there, Will needed to find a stable relationship. However, the show was still attracting high viewership, so it kept getting renewed. The quality of the show got so bad that it was nearly unwatchable. In order to keep the show going, Grace's marriage had to be broken up, introducing a painful, and very, very unfunny plot about infidelity and divorce. It morphed from a consistently funny comedy to a soap opera with occasional chuckle-worthy moments. A sixth season was always going to be stale, but a seventh and eighth? Will and Grace ultimately deteriorated so badly because the original premise, the foundation of the show, was violated.
Even my favorite sitcom, Parks and Recreation, fell into the trap of too many seasons, though every episode through to the finale was well-written and hilariously funny. The sixth season finale didn't tie up all of the loose ends, but it reached a satisfying and natural conclusion. The Parks and Recreation department had splintered apart, for good reasons, but nevertheless had done so and the basic story had come to an end. The seventh season was essentially a season-long finale, wrapping up every storyline in laborious detail. There was something slightly sad in such a clear wrap-up because it carefully boxed each character into a definite future, limiting the scope of their future possibilities. For the sake of the story, an extended bonus finale in lieu of a full season would probably have been better.
In the case of The Big Bang Theory, about to begin its tenth season, the show is suffering terribly from the pacing problems of too many seasons. The essential premise of the show is four nerdy guys who have to grow up enough to be able to sustain long-term relationships with women, but three of the four characters had already achieved those relationships by season three. The end goal of the show, similarly to Friends, is for Leonard and Penny to make a serious commitment to each other - that was established in the pilot - and both Sheldon and Howard were set up with Amy and Bernadette early on, leaving only Raj to go through the motions of romantic entanglements. This issue has meant that the writers have had to repeatedly break up the couples to sustain the arc of the show. The show's biggest weakness is that, despite having a stable central cast of seven characters by season four, not all of the potential relationships have been fully exploited. Though Sheldon's friendships with Leonard and Penny and the friendship between Howard and Raj have had significant attention, Amy and Bernadette continue to be satellites, relating to the other characters via their respective boyfriends or Penny. A greater focus on friendships and careers might enliven the show and help it along. But, by the seventh season, the series was already feeling pretty tired; at this point, it's about time to mercifully put these characters to rest.
In the end, it seems that the vast majority of well-written sitcoms have a healthy life of about six seasons, seven at a stretch. The Big Bang Theory doesn't really deserve the intense critical backlash it's received in the wake of insane numbers of awards and nominations. It's a pleasant, sunny comedy, peopled with characters that range from gently annoying to ineptly endearing. It's not going to make profound political points because it studiously ignores politics. It's cotton candy, but like cotton candy left out too long, it's quickly calcifying.