I disagree with your description that 'plenty' of films have gone the route of casting blacks or other nonwhites into 'white' roles. I don't think there is an equivalence between whitewashing and nonwhites being cast in some white roles, and especially when you factor in the entire history of Hollywood.
In no way would I EVER, EVER claim there was an exact equivalence there. That's an argument I despise. Any fair and honest person will acknowledge that there is a long history of inequality in casting that needs to be corrected, and it will be a long time before that balance is corrected enough for there to be any kind of equivalence. What I meant to say is that things do seem to be getting better than they were, that there is enough casting of nonwhite actors in traditionally white roles to give us hope that the imbalance is on its way to being corrected. I'm not saying it's proof that the problem is solved; I'm saying it's an example that needs to be emulated and encouraged if we're going to solve the problem.
However, recent developments have made me less forgiving about the casting of Cumberbatch as Khan. In isolation, it could be defended or at least forgiven, but looking at the big picture, it feels like part of an unfortunate pattern in the industry. First we had the leads in The Last Airbender
. Then we had Khan. Coming up, we've got the Lone Ranger
remake with Johnny Depp as the first live-action screen Tonto in history who hasn't
been played by a genuine Native American/First Nations actor (Depp's claims of partial native heritage are unconfirmed). And now I've heard that Michael Bay, who has a history of questionable racial portrayals in his films, has cast the decidedly non-Japanese William Fichtner to play the Shredder in his upcoming Ninja Turtles
movie, which is just incomprehensible. At this point it's starting to become a disturbing trend, and it's most unfortunate that a Star Trek
movie of all things is a contributor to that trend, even unintentionally. I don't want to accuse STID's filmmakers of deliberately contributing to the exclusion of nonwhite actors; I don't believe that was their intent. But it doesn't seem they tried hard enough to buck the trend.