In many forums, it seems more like a racism debate. It's not PC to complain about a "black" actor taking a role that may traditionally have been filled by "white" actors, but there is a fierce anger about casting a "white" actor (man, I hate this discussion) in a role that was traditionally held by someone of colour (and I don't include Spanish RM here).
I do recall some bitching about Idris Elba in Thor
The most common response to the complainers was not to denounce the complaint as politically incorrect, but rather as idiotic given the universe in which these movies take place. You can accept a magic hammer, but not a Norse god who looks African?
The same observation applies here. Khan has MAGIC BLOOD
and we're nitpicking skin color?