https://www.indiewire.com/2022/08/blonde-backlash-accent-hollywood-1234750773
Accents are changed by actors all the time. It is something that you can change with proper training. Obviously, there are masters of changing accents like Gary Oldman and people who are notoriously bad at it like Kevin Costner. There are also people like Anthony Hopkins, and Patrick Stewart who just seem to keep their iconic voices in place no matter the role.
That said, you have to be living under a rock or intentionally nit-picking to claim that this is somehow a race issue. Anthony Hopkins has already been criticized for his performance in Armageddon Time for being the only person in the movie who sounded British in a movie about New York.
Michael Biehn's career almost failed to take off due to a thick Southern accent. He almost lost the role of Kyle Reese because of his struggles to change his accent.
I mean, do we really think that Daniel Day-Lewis would have been the right choice to play Lincoln or Daniel Plainview if he couldn't alter his accent? Would Gary Oldman's performance as Drexl Spivey have worked if he sounded like a dude from London? Would Idris Elba be as great a contribution to The Wire; or, would he have stuck out like a sore thumb being the only eloquent British dude on a show about crime in Baltimore?
I mean, sure, I can sympathize with people who are actors who also struggle with accents. Some people simply don't have the ear for it. I've held auditions and participated in auditions where it's been a problem and it was downright heartbreaking to have an actor leaving knowing that he or she didn't get the part because of the accent. Still, casting decisions have to be made with an understanding of just how far an audience is expected to suspend disbelief. If you're making a Civil War movie and you're casting Robert E. Lee, no matter how good the actor otherwise is, you can't have Robert E. Lee sounding like he just got off a ship from New South Wales. Hell, you probably don't want him to sound like he's from New York or Boston.
Again, I'm not trying to claim that taking on another accent in a manner that's convincing is easy. I'm also not denying that actors with specific kinds of voices have had lucrative careers without ever having changed their voices even when their accents didn't make sense for the character. These are usually people with iconic voices though; and, those are rare to come by. I'm also not denying that certain accents are harder to do and harder to move away from.
Still, this is simply part of the job of an actor. Changing your accent isn't a talent that you need to be immensely successful; but, not being able or willing to do it will reduce the roles that you can play. You don't need to be able to sing to be a successful actor; but, if you can't, you can't expect to get many roles in musicals. Stage combat isn't something that you have to be good at; but, you're not gonna have as many opportunities for roles in action movies.