Trey and Matt have said in interviews that over the course of the show they realised they are more like Cartman than the other characters, which is why they stopped using Stan and Kyle as a mouthpiece as much, and we saw more of Cartman representing their viewpoints. For example in the cartoon wars episodes.

And also, not everything Cartman says is evil. Theres 26 years worth of South Park, hes said hundreds of hilarious and relatable things where he definitely had a point.

How did they fix Stans voice. Are they using AI?