South Park lost its luster before the political leanings came into play in later seasons just because it lost its comedic wit when it traded in the gross and edgy humor for Greek epics and goofy, randumb slapstick humor. The best example of this is Eric Cartman going from being some edgy fat kid to a pudgy Sauron. It's really hard to pinpoint what exactly caused the decline but what it ultimately boils down to is an analogy I just cooked up of a sharp pencil becoming dull after mileage.
As for other shows, I struggle to think of any since I have a decent radar for forecasting what's going to be pig shit and what won't, with rare exception.
The Simpsons comes to mind since that went down a death spiral after season 10 or so. Where early Simpsons episodes were actual stories with a point to be made, modern Simpsons are ten minute what-if vignettes that are largely of no consequence and hold no stakes whatsoever. Before that, the Simpsons were still full episodes with jokes that could get a laugh, they didn't have anything strong to say. It didn't help that the setup to jokes gradually got more and more tortured for weaker payoffs.
I never read the Game of Thrones books so the show didn't lose me until the final season where D&D couldn't even keep the continuity of events cobbled together anymore.
The Walking Dead after Rick Grimes left the show. The actor wanted to leave the show on the promise of making an eventual return so they had Rick helicopter-rescued out of the show, so now there's no one left in the show to follow anymore. The series had been building up to Rick passing the torch onto Carl and Carl's pointless, anti-climatic death was the penultimate nail in the coffin. And now Negan of all characters is one of the show's leads, which is just fucking stupid. A real person like Negan would've been dragged to the center of Alexandria to be publicly shamed and beaten by everyone he ever wronged before being executed. Keeping him alive to prove a moral point was just dumb.
I mostly agree. I loved The Punisher show. But you'll really, really hate season 2. Season 2 pissed me off but there was enough good about it to make up for it.
This one I totally forgot about. Season 2 had lots of annoying hangups that I was close to giving up on it. You could do a lot with a character like Punisher, Hammerhead from Spiderman is also a Punisher villain if I recall correctly.
Petty Dreadful is another one that turns to shit after season two, the things that mattered in season one were made irrelevant and nothing up until that point mattered and all aspects of it were inferior.
Netflix Dracula was great until the third episode, where the plot gives up on intrigue and it turns out, that despite being an immortal monster of the night that eats people and makes monsters from the corpses he eats, isn't even hurt by sunlight. In that instant, the one thing that kept him in check from global genocide should he whim it was made irrelevant, giving him carte blanche to commit whatever atrocity whenever and wherever he felt like without fear of retribution whatsoever. Then Dracula just chooses to die anyway because the script calls for it even despite the fact he has no reason to want for death and went into quite a considerable amount of effort into continue living as much as possible until that point. I have never seen such a quality show take such a hard nose dive at the last second like that.