Re: This is the BBC making a quota
... Idris Alba for James Bond
(I hope that was deliberate, "alba" meaning "white")
Idris Elba is a fine actor, and I think he'd make a splendid "00" agent, but he's not Bond. James Bond is a specific (albeit fictional) person who happens to be male and white, and there's no reason to cast him as anything else.
Doctor Who is different. The Doctor has always (Joanna Lumley and Lenny Henry notwithstanding) been played as a character who is male and white, but is an alien whose appearance occasionally changes, sometimes quite dramatically. There's no reason for the Doctor not to be played by someone who is non-male or non-white. There is nothing in the canon that says that this can't happen (and some things that suggest it can) -- only audience expectation.