All good shows have to end sometime, but I still think South Park has enough life left to last for another season or two. Ending the series now just seems a little abrupt.
Of course if they did end it this season, it could mean Matt and Trey are setting the series up for another movie. I really hope that isn't the case since Bigger, Longer, and Uncut wasn't that great in my opinion. Still, it's up to the development team to figure out how they want to end it.
I wouldn't be surprised if they just gave a middle-finger to the fans and ended it mid-sentence like the Sopranos.