Tell you what...I'll agree that this casting is ok if YOU'LL agree (and MEAN it) that it would be ok for a white actor to play Booker Washington, or Kunta Kinte, or any other black character.
It's not necessary to concede a nonsensical point in order to secure your "agreement" to anything: people will do as they will do, and non-traditional casting is already here to stay - it's not even a novelty any more.
Patrick Stewart, of course, played the lead in Othello nearly fifteen years ago; because the ethnic contrast between that character and many of the others is important context to that story, the producers simply cast black actors in the other parts.