It is a truism that in the history of American television that there have been very few good shows. It is also a truism that many good shows, perhaps most of them, have gone down hill later in their run. Often this is not so much a case of going bad all at once--there often isn't a "jump the shark" episode--but more a case of a gradual deterioration in the show's quality. And sadly once a show has deteriorated in quality, it often stays on the air for quite some before ending its run.
There are no shortage of examples of shows that have outstayed their welcome. One for me is The X-Files. For its first several seasons, The X-Files was one of the best shows on TV. That having been said, in its sixth season the show began to show signs of wear and tear. The show was still capable of producing some great episodes. That season saw the two part episode "Dreamland," one of my favourites in which Mulder switched places with Area 51 agent Morris Fletcher, "How the Ghosts Stole Christmas," with Lily Tomlin and Edward Asner as a pair of humourously malicious ghosts, and "Terms of Endearment," with Bruce Campbell playing a demonic (literally) father. While the season produced some good episodes, it also produced some very bad ones, such as "Drive (a man suffering from a deadly disease takes Mulder on a drive)" and "One Son (yet another mythology episode)." The seventh season saw the show decline even more in quality. True, there were some very good episodes ("X-Cops," "First Person Shooter," and "Hollywood A. D."), but the merely mediocre and even bad episodes began to outweigh the good ones. The decline of The X-Files continued with the eighth and ninth seasons. Indeed, the absence of Fox Mulder from the majority of episodes definitely had an impact on the series.
As I see it, it would have been better if The X-Files had ended with its sixth season. It was that season that the show started its dramatic decline in quality. Indeed, I have to wonder why they chose to continue the series when David Duchovny decided to leave. The show simply was not the same without Fox Mulder in nearly every episode. It would seem that The X-Files was one of those shows whose success depended largely on its main character.
Another show I feel has outstayed its welcome is The Simpsons. Quite frankly, for its first many seasons I would say that The Simpsons was one of the greatest TV shows of all time. It was simply brilliant. And it is amazing that it remained as good as it was for such a long time. I suspect that the decline of The Simpsons began before the twelfth season, but for me it was during that season that I first seriously thought The Simpsons was not as good as it once was. I can even remember the episode in which I first thought that The Simpsons was no longer a great show. It was the episode "New Kids on the Blecch," in which Bart, Milhouse, Nelson, and Ralph become a boy band. I found it unrealistic that Bart would ever consent to being part of a boy band, and I found the episode's conclusion rather hackneyed and unoriginal. Quite frankly, I think The Simpsons had fallen far short of what they once were.
Now I do think The Simpsons has improved since then, although it is still not nearly as good as it once was. In fact, the show is not nearly as good as The Simpsons Movie. As much as I love The Simpsons, I have to wonder if the show should not have been put out to pasture several years ago.
At least both The X-Files and The Simpsons had long runs of great episodes. While I know that there are many who will disagree with me, I don't think that can be said of The Cosby Show. I was a big Bill Cosby fan when the show debuted in 1984. And I thought that the show was simply brilliant for its two seasons. Sadly and rather unexpectedly, the show went from being uproariously hilarious to being a rather standard, family sitcom. Indeed, it surprised me as to just how unfunny the show had become in later seasons. Amazingly, it would run another six seasons, four of which it remained the number one show on television. If they could not have maintained the first two seasons' quality, I would have just as soon the show would have gone off the air long before it did.
I suspect the reason that shows, such as The X-Files and The Simpsons, sometimes gradually decline in quality during their run is rather simple. As time goes by it becomes harder and harder to come up with original episodes that fit within the concept of the series. At the same time on many shows, many of a show's writers and directors will move onto other things. I suppose that one can simply say that shows grow old. As to why some shows suddenly go from being good to bad, the way The Cosby Show did, that is more difficult to say. I don't know the entire, intimate history of The Cosby Show, but perhaps they changed staff suddenly between the second and third seasons...
As to why shows remain on the air long after they've declined in quality, why they outstay their welcome, that reason is simple: ratings. The X-Files maintained good ratings nearly until the end of its run. The Simpsons still gets good ratings. I know that I watched both shows long after I thought they had gone downhill. I rather suspect this is true of many viewers. Particularly with cult shows, viewers aren't eager to give up their favourite shows even after it is clear they've seen better days. Beyond viewer loyalty, there is also the fact that many viewers are not particularly discerning in their viewing habits. I am sure many of us can think of examples of shows that never were good, yet they enjoyed healthy runs. In the case of good shows, I think most producers should follow the lead of David Chase. He ended The Sopranos while the show was still good.