I hadn't thought of it like that. That would definitely fit the theme of the episode. Stan turns another year older, and all the sudden, the stuff he used to like, he now hates, and thinks it's shitty -- just because he's a cynical asshole now, and the only thing that's changing is YOU. South Park is still the same foul-mouthed, gross-out, off-the-wall show it's been since day one. I don't think it's stagnating at all.
Personally, I think South Park is still in its prime. This season has had its share of AWESOME and average episodes, and definitely hasn't showed signs of its looming death. If Matt and Trey did decide to end it, I'm sure it'd be because they're burnt out, not because the show is suffering.