People felt like they were being preached at rather than entertained.
Then this is likely as true an indication as any that sometimes racism and stupidity are indistinguishable.
I think some wrap their arms around things not on the basis of its merits but because they've been triggered by haters.
Insert "white establishment" in place of the word "some", and you have stated one of the concepts that triggered the civil rights movement in the mid-fifties -- "haters" (racists), refusing to accept on the basis of merit rather than "entitlement". This is also what triggered the "oscarssowhite" movement a few years ago.
The problem that you and others have is the believe that if a racial minority or woman or trans person is given a role that could also have been played by a white actor, then there needs to be a reason, if not, then why not have a white actor play the part? In other words, white is the default setting unless there is a reason to deviate. Then, when the reason to deviate turns out to be for the sake of diversity, there is an outcry against "diversity for diversity's sake". Do you realize that the alternative is white (male) actors only in roles unless the character is written to be a woman, minority, trans, handicapped, etc?
There also seems to be a belief that a number of minorities in a show means that these actors were not chosen for their suitability for the role and acting talent, but for their skin color or gender. This is the very essence of racism and sexism. You do understand, I hope, that choosing the right actors for the part, even if the choice is a minority, is in the best interests of the producers. Doing otherwise would be foolish. Now, sometimes there are casting mistakes made, meaning the actor chosen may not turn out to be right for the given role, but if this happens, try not to think of those mistakes as being related to the actor's race.
This is especially true when you're dealing with properties that go back many decades. These franchises are like a modern mythology and they carry over a lot of quasi-religious baggage. So an ethnic or gender flip can be treated almost like a "black Jesus" or "black Santa Claus". (Dr. Who is dealing with that the most at the moment.) I think this issue is far less pronounced with the scant few originals that come out of Hollywood because there's no preexisting iconography. But I think some look at these preexisting characters as almost like it's a political office in which there is some societal obligation to cycle women and different races through there so each can get their turn. In that case I await a male Wonder Woman with great anticipation.
In the case of fictional characters, many of them were a reflection of the intolerant times in which they were created and sometimes the personal prejudices and beliefs of their creators. So swapping them out for minorities or a woman should not be a big deal.
Re: Wonder Woman, once again, this is based on a false equivalency. If women and men were equally represented in superhero roles, then you might have a point. But they are not. Far from it, in fact. Now, perhaps one day, after we have gotten so used to female superheros, we can legitimately wish to see a male in a role traditionally played by a woman. Wouldn't count on it being WW, though, her gender is part of her story.
Try to not think of white (male) as the default setting, and it might be easier for you to get what I'm saying here.
BTW, Santa Claus is a fictional creating and can be any gender or race, and as cultcross so correctly and concisely put it, Jesus surely was not white.