Netflix released the original show “Supacell” with new and original characters, but in the comments, right-wingers are complaining that the protagonists are black. The film quiet place day one was released and had a black protagonist. The character is new, original and coherent. Quiet Place is set in the USA black people exist in the USA, so what’s the problem? If they are going to complain anyway, then why do they ask to make new characters?
The trouble is, you’re looking for internal consistency to their arguments. You need to look at the theme of the arguments: namely, they don’t like the “black people exist in the USA” part.
People are complex, and sure, some are just racist, but I don’t think it’s that simple.
Hatred isn’t nearly as powerful an emotion as fear. White people complaining about non-white main characters aren’t necessarily racist; it’s more likely, I think, that they’re afraid of losing the only consistent part of the white identity: being the “default” race.
That’s racism with extra steps
Racism is not just a belief or a behavior, but a system of disenfranchisement. The system of reinforcing white as the “default” is part of racism. Someone who harbors no specific beliefs about other races, but acts to support the inequality of racial groups is still being racist.
Or as another user put it more succinctly, “racism with extra steps.”
Hatred often stems from fear and your example is in line with that.
They want someone below them to oppress and/or feel superior to; if those people were completely erased, they wouldn’t be happy about that either, unless they could find new victims.