The definitive answer is Supernatural. Shambled on for another 10 seasons as the reanimated husk of a really good that was written to end after season 5.
It's nice that you were able to enjoy it. No one can invalidate your enjoyment. But for me, it was a shameless capitalisation of characters I'd grown to love. A decision driven purely by money, quality and coherency be damned. The cinematography got sterile and lost the gritty, almost horror-film-esque feel of the earlier seasons; the storylines got goofy, Jared Padalecki's acting somehow got progressively worse; and worst of all, the show got the most cliched, predictable, fan-service ending imaginable. Those last 10 seasons felt like a bad fanfic brought to screen.
Fortunately, because season 5 was supposed to be the end, it works as a natural jumping off point. I can just pretend everything after that doesn't exist.