You can always tell when a TV show has played its course. It happens when they start spoofing other TV shows and movies.
Basically, when you're out of ideas you start saying, "Let's do the fight scene, except have Jimmy and Timmy do it". Or "Let's re-run a Mel Gibson torture scene, except use Santa". Or any of hundreds of different pop references to various TV/Movies.
South Park is, and has been brilliant. Killing Jesus just to set up the "Why not have today in his honor" line is awesome. Or Stan's rantings in the Biggest Douche in the Universe. It's all true satire and is brilliant.
But when you put your characters in someone else's "famous scene", it cheapens the originality of that scene and its creators, while doing nothing to further your originality and humor.
I wish they'd do an episode on lame TV douchebags who are so starved for content that they have to resort to co-opting other scenes they've seen in the Movies.