You put it really well with the characters disintegrating. Now they're mostly all stereotypes to further the plot about a pop culture conspiracy, the formula for nearly every episode. Yes, Cartman had a formula of being an impudent prick in most early episodes, but it was real life, not like being the mastermind of a conspiracy about whatever celebrity/topic is the flavor of the moment.
It may have been amusing at first but now it's just pandering or even blatant advertising (as with PewDiePie, in my opinion; again, I know little about him and have nothing against him, since South Park fans think if you didn't find it funny, you must be angry at PewDiePie and therefore maybe its being "ironically shitty" was a success. They just put him in there, forgetting any jokes, or real sense of purpose. I think either Trey or Matt even commented it was because their son liked him...ok.
I guess in earlier episodes, it managed to be zany and hilarious while some sort of logic or real-life zeitgeist ruled. It wasn't just "Boom, what if Iggy Azalea's ass is a snowman! Laugh!" out of nowhere. And when they did miss on jokes like that, they didn't keep repeating them in hopes the repetition would make it awkwardly funny, make Iggy look foolish and talentless, or turn into an "epic meme." That seems like the era we live in but it's still less funny. When they ripped on a celebrity, it was smart. Example: They lampooned Rob Reiner scarfing donuts and being morbidly obese to show the hypocrisy of also being an anti-smoking advocate and the attitudes surrounding it. They didn't just randomly wedge him into an episode as Jabba the Hutt to put him down if he was the relevant guy at the time. Even when it was less polished and "shittier," I miss episodes about real experiences, recorder concerts, going to the rainforest, etc. because the characters actually developed and had real personalities. Now if they go to the rainforest it has to be for a segment parodying an irrelevant show that already no one really cares about, and even much less so in just 10 or so years from now.