I think back in the 60s Stan Lee, Jack Kirby and the other big writers were more focused on making interesting heroes then interesting villains, with a few big exceptions like Doom, Red Skull, etc. Back then heroes with real flaws was revolutionary and so that was the focus, I don't blame Stan Lee, Jack Kirby and the rest for focusing more on making interesting heroes and having some of the villains be one note. Plus I think people tended to see villains as just "the bad guy" rather then their own characters, back in the 60s.
The problem is, at this point the heroes have been well established and a lot of villains have never progressed beyond their one note characterizations from the Silver Age. I like Stan and Jack, but respecting their legacy is the way to go, not being so slavish devoted to it, that we can't have any change.
I usually don't like big retcons, like Spidey no longer being married, but I don't mind smaller ret cons, like ones that give villains a better back story or more characterization. Or even have an event that changes a villain, like what happened with "Coyote" in the Daredevil book. I think you can take a kinda of lame villains and try to make him interesting with a little effort. I think there are tons of B-list and C-list villains you revamp either by giving them more interesting back story or having an event happen to them that changes their outlook. Also if you make a change, you have to stick with it and more then one writer has to stick with this change. Hickman had Wizard go through a mental break down recently, but we haven't gotten a good explanation of why Wizard had a mental break and every other writer besides Hickman seems to be ignoring it.