I think most tv shows have a shelf life of awesomeness...and that generally translates to 4-6 seasons, before things start to become stale or repetitive. There are great exceptions that, of course.
One that comes to mind would be The X-Files. I love every second of it, but there's no doubt it began to decline in later seasons and the movie was even worse. Then they tried to bring it back and it suffered badly.