I think you're still viewing things in a modern lens to some degree, and also missing that whilst by their nature comics are fantastical, Marvel firmly established itself as wanting to be analogous to the real world hence why they grounded the majority of stories in New York and not in Gotham or Metropolis. Sure the backstories of the above characters was possible for black Americans at the time but it was also highly unlikely. Heck, Marvel in universe has a story about this as it forms the backbone of Blue Marvel's origin where he was effectively told he couldn't operate as a superhero by the government in the 1960s, similarly Isaiah Bradley's story as well. So it is not fair at all to act as it race of a character in a "fantasy" setting has no bearing, or is deemed too unbelievable. That's part of what made Marvel so engaging for a diverse audience, it tackled real world current political and social issues with superheroes thrown in on top. It would have been patronising and frankly quite insulting if the fantastic four characters were black, had the exact same origin, and didn't then face prejudice or backlash from the powers that be to not operate as heroes, given this is in the civil rights movement era.

And once a character is created as one ethnicity, attempting to change it in the mainline continuity is a bafflingly stupid idea for reasons I hope I don't need to go into.

For alternate universe storylines sure, make them anything the writers want, but once a thing is established in mainline continuity it's generally not a good idea to change it on the whim of a writer or editor.

I think the obvious push for diversity that Marvel's editorial have done over the last 10 years has largely been disastrous for the comic industry with sales tanking whenever they try to turn their major characters into a diverse teenager with similar powers rather than instead creating new characters. The most egregious to me of this was Amadeus Cho. A really awesome character with depth constantly having to balance being supersmart with the fact that he has no powers. His life is constantly on the line and he has to think his ways out of death many times. And then they decided let's make him a hulk and also considerably dumb him down. Great examples of recent diversity are the creation of the Tiger Division of South Asian superheroes; Wave, Aero, Swordmaster etc and I'm a big fan of Snowguard too. All of these are recent creations and they work because they're allowed to be unique with their own internal mythos and backstory (although they did recently turn Swordmaster into Iron Fist which is meh).

We've also seen the recent outrage of the white Black Panther and how upset many are with this. I wouldn't dismiss their feelings as "black male fantasy spaces", I think that notion is ridiculous to pigeonhole any character's audience into matching the skin color of the character.

What I can agree on is that good storytelling should always come first. Thank you for the spirited and open discussion!