Plot. Story happened. Character development happened. That's the answer for everything from the face turning villains to Silver Surfer. The characters are not defined forever as one thing, they can change, that's the point. There was a time where almost everything always reverted to status quo, with a few exceptions, sure. And it still happens. But what you're seeing I think is that audiences and the writers are wanting to have some character and plot developments stick, have the characters go through transformative arcs and not just undo it all for the next writer. The 'illusion of change' is falling out of fashion, at least with some writers/editors. What you are describing is still not 'villain ascendancy' because they are
not villains any more. Or are at least on the path to not being villains. While I understand why a lot of readers still refer to them as villains given their history, at the moment in the books, their actions do not match the label of villain. They are not doing villain things. Maybe their methods are a bit dubious at times, but they are not trying to cause harm. It sounds like you are assuming it's a ploy rather than a genuine change. And while I think that is the case with Fisk, at least in the case of Loki and Ock, possibly Magneto and Doom, it is a genuine change, they actually do feel bad about at least some of the things they've done in the past, and want to make changes. They're not at like, outright hero levels, they're anti-heroes, and will likely stay more gray area characters, but they're still not villains. All of those characters have vastly different backstories and motivations, and they can't all be lumped together just because they all were created as villains, they all had different paths. And just because these characters are doing that, it doesn't mean it's some kind of universal trend. They are a minority, if you look at the big picture, they just are getting more focus than some previous face turns who were lower tier characters (Songbird and the like) or they happened so long ago it's sometimes easy to forget they started out as villains (Scarlet Witch/Quicksilver, Rogue and the like) There will always be unrepentant villains to come on the scene who the good guys have to stop, Red Skull or Carnage are unlikely to ever turn face, and new villains can always be introduced.
and to the point made above, I have seen people who have made great changes in their lives for the better, (this will seem a bit random but it comes from the heart, and he's just a guy, so it's relatable, so here, a link to a youtuber who went from attempted murder to successful aquarium guru and really nice guy who has done lots of good:
https://www.youtube.com/watch?v=Rf9IGQ8WAOM ) so I don't find it unbelievable that certain villains can come to realize the error of their ways, I think it would be unrealistic if at least a few of them didn't. Yeah, they start out from a worse place than would happen in real life cus drama, but that's just comics for you, everything is bigger than real life. Not everyone will want to change, most won't, but some will. I think it's aspirational and encouraging to be reminded that you can always do better than your worst moment. It seems like a horribly pessimistic view to assume someone should forever be defined by their worst actions and can never move on from that.