A brief, general overview for people confused on the seeming double standard for race swapping fictional characters:
You cannot discuss this subject in good faith if you're not acknowledging that casting takes place in a world built on centuries of double-standards that work for the benefit of white people and to the detriment of people of color, including systemic disenfranchisement from off-screen and on-screen roles.
Simply put, European colonialism has allowed white people to position themselves in media as the universal protagonist, the everyman that everyone else is supposed to relate to. However, this is a double-edged sword. Sure; it means an abundance of white characters, and thus plenty of opportunity for white actors, but it's at the cost of any actual sociopolitical meaning to the trait of whiteness (at least, any meaning that most white authors have the balls to tackle). I would wager the majority of characters aren't white because they need to be. They're white because whiteness is "normal" and thus serves as a shortcut for an author to focus on more interesting character traits.
On the other side of the coin, notice how people say that minorities need a reason to exist in certain genres, how it's ridiculous that a black person dare step foot in Europe before the 1900s? Yeah; that ridiculous gate-keeping means that, for better or worse, ethnic characters tend to be more tied to or indicative of a unique culture or perspective that you simply can't swap white people into without being completely tone-death and ignorant.
This is why a black Captain America is fine at the same time that a white Black Panther is stupid. It is also why outrage about casting non-white people as characters that were previously defined as or presumed to be white shows racial bias, because it has to presume a world of racial equity, which is also stupid.