If Helen of Troy can be portrayed as Black simply because she’s a fictional character, then Black Panther can be played by a Mexican actor for the exact same reason. Both are made-up characters, so the “it’s fiction” defense should apply equally to everyone. Any character should be able to be any race if we’re truly going by that logic.
If a character’s race truly doesn’t matter, then no one should get upset when the race gets changed in either direction.But if people lose their minds when a beloved Black character is made Mexican, then clearly race does matter to them. In that case, no character should deviate from its original cultural and ethnic intention.
You can’t have it both ways. Either race-swapping is always fine because it’s all fiction, or the original identity should be respected across the board.