When it comes to a good TV show turning bad, most often the biggest enemy is, quite simply, time. Most shows will find their peak around the Season 3-4 mark, and while they'll run on a lot longer, it's often in a state of creative decline.
Take The Simpsons, for example. There's not one decision where you can pinpoint the exact moment the show's quality was ripped in half: yes, Principal and the Pauper was a misstep, but there are broader issues around the writing of Homer, Lisa, Flanders, and so on that all combined to the gradual decline of the series.
For some shows, however, the decline can happen in an instant, or in the time it takes the Fonz to jump over a shark. Sometimes one bad decision is all it takes, and that show you loved is suddenly a shadow of its former self.
10. Prison Break - Having A Second Season
Prison Break is a good example of how sometimes less is more, and that not all TV shows need to come back for another season. During that first season, there was a very clear concept - the titular prison break - which was executed brilliantly. It was fresh, exciting, and among the best shows on TV at that time.
And then the prison break succeeded, so what came next?
That really could've been the end of the show. It's called Prison Break, and they've broken out of prison. Creator Paul T Scheuring has revealed that he had the first season mapped out in total detail, but that was it. He knew everything that was going to happen, every plot and character beat, but nothing more - and then the network asked him to bring it back, and he agreed to "stretch it out".
That's a fitting line, because while Season 2 was still pretty good, the show got progressively worse over its original four-season run. The plots were stretched thinner, the situations became more ludicrous, and it became rather terrible TV over time. Sure, you can make a case for ending it after Season 2 instead, but the first is so far above everything that would come after it's really not worth it.