They really do. The sweet spot for most shows is 3-5 seasons. I've found that most that go beyond that have to resort to ridiculous crap to keep the story interesting. It would be better if they got in, told their piece, and left it at that. I'm looking right at you Weeds, Dexter, and True Blood.
It's what I really liked about The Tudors. Another season of it would have probably ruined it. It ended in a satisfactory way for me and never got to a point of stupidity. Spartacus was the same. They had a story, they told it, and then it was done.