South Park kids are overmature these days, even more mature than adults, as a result the show has to focus on adult topics, like politics, and real social life. While in the old days, kids are innocent so when it comes to adult things, they'll have no idea so they do all kinds of weird things, and that's what makes the show funny, to audiences all over the world.
They try hard to make plots complex, adding in a lot of turns and suspenses. But they forget that it's a comedy, and it should never have thick plots because if everything is closely related to the plot, digressive and random humor will be stifled. And the worst thing is, Trey's been aging and he is not really capable of writing complicated stories, that's why recent season closings are all messy.
To sum up, the show has been greatly declined in creativity and originality, if the situation stays still like this, I'm sure the show will come to its end in a very near future.