I think the best examples are Doctor Who and Marvel Comics. Doctor Who is almost 60 years old, and it had plenty of ups and downs over the decades, but it's only now that it's been damaged seemingly beyond repair, with plot developments that will be difficult if not impossible to repair, and, worse, a thoroughly alienated fanbase. Similarly, Marvel had plenty of periods of low quality during roughly the same period of time (if we date modern Marvel from the birth of the Avengers and Spider-Man), but it's only now -- ironically, while movies based on their material are the biggest entertainment juggernaut in the world -- that the quality is so damaged it's practically on life support.
There's something uniquely corrosive about identity politics. I think it may be because it's nearly a religious conviction. The people responsible will not abandon what they're doing to these properties no matter how badly it affects the bottom line. The only hope is for them to collapse completely and, maybe, get resurrected by someone who still cares about the core values of the original product. But I can't even think of an example where that happened.