Am I the only one who thinks The Simpsons qualifies? No??
It's a tough call. On the one hand, I think America's yellowest family has become a terribly spastic show. Instead of focusing on the relationships the Simpsons have with each other and the other members of their town, it's become a constant stream of pop culture jokes. It's snarkier than it used to be, and a lot less timeless.
But I don't know that I want the show to be canceled. I just want it to somehow return to that charm it had in earlier seasons. But I don't know if it can...so would it be better to cancel the show or just keep churning out weak episodes?
That's the only show I can think of, to be honest. I don't watch TV in "real time", but through Netflix, so by the time I get to a series it's often been canceled. (For example: I'm only about six episodes into the first season of Glee and Rome; the first series is now in the third season, I think, while the other ended in 2007.)