2 things wrong with this line of thought.
Black Panther is entrenched in African mythology and culture. As such black actors are crucial. Peter Parker is not entrenched in any white culture and his stories would stay the same regardless of race.
Diversity means more than just the majority. The current majority is white, its been that way for a very long while now. When people say diversity they clearly mean less white and more of any other race as adding more white people to anything is not diversifying its homogenizing.
And I guess a final one is that currently your definition of "diversity" happens all the time. White actors are constantly taking the roles away from minority actors or are always considered in places where it wouldn't make sense. Look at Gods of Egypt or any biblical movie. Whitewashing is very real as is pandering to a demographic by changing their race just to do it. That complaint is almost always used when a black or other minority actor takes on a historically "white role" yet it always ignores the massive amount of times white actors do the same.
