My take on this.
I find Idris Elba to be a great actor. I'd love for him to play a superhero or hero in general. He has the look and he has the pull that's necessary. If someone wants to make him Black Panther or Luke Cage, I'm all in as long as its a quality product around him.
However, I don't want to see him in this role. For me, certain characters are so ingrained as a certain race or a certain look that I'd rather a lesser talented actor who has the look play them then a better actor who doesn't mesh with my outlook on what that character should look like.
In my mind, Batman should be a white male in his 30's with darker hair, who can pull off the pretty playboy genius while also having that darker silent angry side that comes out when he kicks someone's arse when necessary. That's the image I get off when I think of Batman and that's the image I believe most people get and want to see on screen.
As for minor roles being changed some. Doesn't bother me in the slightest. I don't mind Elba being a black Heimdall. I don't have this strong prior connection with the character that he has to look a certain way. Heimdall was only in a couple of scenes in Thor and his role didn't make or break the movie or the story. Same with SLJ as Nick Fury. For most people, SLJ is Nick Fury.
I don't mind minor character's having their race changed as long as it does solely for the race of increasing the quality of the final product rather than making a statement or pandering to a demographic.
Minor and Side Characters, it's ok to do something like this with them because their source material is not as stringent and is pretty flexible. They aren't as important.
Trying to do something like this with core and legacy characters like Batman, Superman, Wonder Woman, Wolverine, Iron Man, and Captain America is insane and you are nothing more than some idiot trying to be PC and make a statement or pandering to a demographic.