Hitting it just right is the real trick, and rarely done.
Anyone got any examples ?
How about The West Wing (non-genre, obviously) or Buffy?
There are good arguments to be made that WW should have ended when Sorkin left after 4 seasons. And other arguments that the show had refound its groove in season 7 and there should have been a Santos or Vinick Whitehouse.
As others have said, you will always find people who think a show lasted too long and people who think the same show should have kept going.