After all, it's not like there's been one, true, unwavering Fantastic Four over the decades. The Thing was originally drawn by Jack Kirby as a lumpy mess; it took a while for him to settle down into the more-streamlined orange form fans know and love. Sue Storm at first could only turn invisible; it was some time before she developed the invisible force fields that made her useful in a fight. For that matter, She-Hulk replaced the Thing on the team for a while. And then there was a popular series where the Fantastic Four turned into zombies. Comics are serial soap-opera fantasies; people change costumes, grow blue fur, die, grow a third eye, come back to life, are replaced by a clone and turn to the dark side. Nothing stays the same. Why, then, is this particular, relatively minor alteration in canon seen as a betrayal?
You could argue that racial difference is more noticeable, or different in kind, than plot-driven death or blue fur or zombiefication. But then, how account for the fact that in the comics characters like Iron Man, Spider-Man, and Green Lantern have, at various times, been black? More, certain changes in racial background or casting seem to provoke little comment. No one, as far as I'm aware, has complained about Scarlett Johansson's casting as the Black Widow on ethnic grounds. Yet Johansson’s background is Jewish. The original Black Widow, Natalia Romanova, has what appears to be an ethnic Russian name; there was no indication that she was originally supposed to be Jewish. Given the anti-Semitism in Cold War Russia, a Jewish ethnic identity would in context be a significant alteration to the character. Why, then, do people care about Storm, but nobody cared about Romanov?
The answer is obvious enough. American racism holds that only certain racial differences matter. Jews, Italians, Eastern Europeans, Irish—all those people are white and can play one another with nary an eyebrow raised. Nobody is worried about whether Sue Storm has exactly the mix of Irish, German, and French-Canadian ancestry as Kate Mara, who has been cast to play her. For that matter, no one would say a thing if the actors cast to play Sue and Johnny, sister and brother, came from different ethnic backgrounds and didn't look much alike. It's only when one is black and one is white that you need to start worrying about family logistics. (And yes, you can find folks doing that on Twitter as well—because getting turned into living fire by cosmic rays is an everyday thing, but adoption is weird.)
"Fans often seem to believe that if a character is changed from white to black, they will no longer be able to identify with that superhero" Aaron Kashtan, a postdoctoral fellow at Georgia Tech who teaches a course on transmedia storytelling, wrote in an email to me. Kashtan adds that this is an example of "unconscious or overt racism"—a point underlined by the fact that the barriers to identification are so clearly arbitrary. Certain different people—Jews, or Irish, or folks with a hide made of orange rock—can be points of identification. Others, especially African-Americans or anyone with dark skin, can't. The issue here isn't staying true to the original. The issue is racism.