Perusing through Netflix selections I've been noticing many classic television shows can fall into one of three categories: shows that were cancelled too soon, shows that weren't cancelled soon enough, and shows that ended at just the right time. Of course, there are also some shows that defy easy characterization--"Family Guy", for instance, started out very funny, got stale and weak, then seems to have improved. There are also the many "filler" shows that NBC stuck into coveted timeslots between "Friends" and "Seinfeld" and "ER" during their '90s heydey--shows like "Union Square" and "The Single Guy" and "Caroline in the City" and "Veronica's Closet" and "Fired Up!" (Yes it's sad that I remember those . . .) But in the three principal categories we have:
1) Shows cancelled too soon--these shows probably had at least another quality season left in them when they went off the air. "Party Down", "Arrested Development", "Freaks and Geeks"--in these cases there was no sign of decline, but they simply either failed to gain an audience or were shuffled into poor timeslots. Generally, there aren't too many of these--quality shows can usually get an audience, and devoted fans can even encourage a smaller audience cable channel to pick it up.
2) Shows not cancelled soon enough--in these cases no one could really be upset that the show was cancelled, since it was clear any further episodes would have served only as a painful reminder that the show used to be good. "All in the Family" is a great example of this--after Mike and Gloria moved out (leaving Archie without his primary foil), the Jeffersons were no longer next door, and Nixon was out of the White House, the show just didn't have the oomph. "Married With Children" is another example--a show that started weak, got pretty funny in an awful way, then declined into self-parody with worse and worse seasons. "NewsRadio" had been solid through its fourth season, but the fifth and final season after Phil Hartman's death was just unbearable to watch.
3) Shows that were cancelled at the right time--this is often a more tricky call--without seeing what additional episodes there might have been, it's hard to say that the show couldn't have churned out additional good ones or alternatively, the well was dry. For this category, the show has to have shown some wear, still coming out with quality episodes but with nowhere else to go with the characters. In this case, you usually go from a show you made a point of watching regularly to it being a show you'll only watch if you happen to have nothing else to do. "Friends" and "Seinfeld" and "Cheers" come to mind--but maybe the best example is a non-comedy, the ABC hit show "Lost". The episodes of the last season were worth watching, as they still had you hooked on the whole "what the hell is going on here?" question that stretched from the first season. But viewers were rightly getting annoyed with the "answer one mystery with another mystery" layering, as well as the whole "someone getting shot before they can reveal something big" and the "characters not pressing for answers the way the audience would" getting formulaic.
That said, when thinking about the shows I try to catch every week--"It's Always Sunny", "30 Rock", "Office", "Modern Fambly", "Simpsons"--I wonder for each of them whether they'll end on a strong note, or have a long, lingering demise.