They were both the straight man and there was no reason to have two of them. Luckily Kyle did change, but theres more to it besides his relation to Cartman.
Kyle became a huge moralfag always trying to do the right thing out of principle, which is why he constantly butts heads with Cartman, while Stan only tries to do right when it negatively affects him or someone he cares about, other than that he doesn't give a shit, and it makes him a boring character because he has no other personality. The only time he becomes interesting is when some of his father's traits appears, like getting drunk and telling Kyle he's a piece of shit.>>96629765
Randy became the go to adult to use for whatever topical shit happens in South Park.
Kyle is the moralfag always trying to do the right thing.
Cartman is always going to be an antagonist.
Butters is sort of like the kid version of Randy, but also has other uses like being Cartman's lackey.