It is simple.
The reason why so many white characters in media transition to minorities is due to a few key factors
- Modern day diversity and PR
- It's easier to change an established franchise with character tweaks vs. making brand new adventures and characters from scratch hoping the new IP sells
- The execs have decided if they can pull it off, it'll lead to giant sales as they're trying to make every consumer happy with a blanket approach to representation
The writers can also do the opposite. Any stories with historically minority focused actors can be tweaked to be 50/50 white/minority. Balance it out. Most media is fictional to begin with (even documentaries are fudged with editing to paint a story), so who really cares if a Chicago ghetto or Wakanda in a movie morphs into having more Whites, Latinos, Indians and Japanese people? Why not? It's fake story and lets be more diverse.
But if they do that, they'll get grilled because some people will try to protect their own turf, saying "No, keep it the old way" when it's just as fake as Asgard.
When some media gets more diverse in character sets, they love it. When their own backyard gets done the same, they complain. Not in my backyard mentality.