I found out about the black Hermione thing by someone I knew came running up to me and basically said they changed a Harry Potter character we should all be outraged, I responded by just saying 'I'm fine with that, Hermione's race is never brought up in the books so she could be anything'.
I think they were shocked as I have previously complained about black Human Torch and black Heimdal, but my thinking is Heimdal is a Norse god so should look traditionally Norse and I would have been fine with a black Human Torch if we had a black Invisible Woman as well. I just don't think its right to change a characters colour or gender if its relevant to the character, but if its not then its fair game to cast whoever you want.
For example:
Black Bond: fine, female Bond: not fine. I see Bond as a male power fantasy and really you can't do that with a woman, it changes the whole point of the character, but a black male Bond can still be the cool guy with the cool car and the cool gadgets that gets all the women that men who like Bond would like.
Female ghost-busters: fine, ALL female ghost-busters: Pandering, ALL male ghost-busters (these days): possibly (not always) sexist. Honestly I would have been fine with 3 female and 1 male if the casting went that way, but all female just strikes me as they are trying to appeal to the feminists, and not the good kind of feminists either.
Dumbledore being suddenly gay: fine, it was never established that he had interest in women or his sexuality at all, Captain America being suddenly gay: not fine, he has clearly only shown interest in women in the past. I demand consistency in my fictional super humans.
Peter Parker as Spiderman: fine, Miles Morales: not fine, I don't like legacy characters like this, but that's just personal preference, nothing actually wrong with it but I'd prefer Miles to be some other hero instead.
Black Hermione: Fine, Male Hermione: not fine, you may as well create a new character because your already going to have to change the characters name and relationship with some of the characters. And I think that is the OP's point, if you want to change a character so much that the character becomes someone else then make them someone else from the start.
I don't mind an increase in diverse characters, in fact I look forward to it, but I don't think the way to go about it is changing existing characters, instead give us new heroes and new villeins for a new, more openly diverse, generation.