The idea of casting Idris Elba as James Bond goes against both Fleming's original premise and 50 years of film evidence. Elba might very well make a good secret agent in the style of Bond. But he cannot be Bond.
Now, Elba's defenders (or should I say, those seeking to cry racist) note that Bond, in Fleming's original books, was Scottish, and that only one of the actors to play Bond (Sean Connery, of course) fit that bill. I also note that Connery is generally considered to be the best-received of the many actors to play Bond, and the Englishmen who have since played the role have typically been considered too camp, especially Roger Moore.
Casting Elba as Bond makes about as much sense as casting me as John Shaft. It would be a naked attempt to diversify a character that was never intended to be diversified. There is a certain continuity that has to be followed: to a certain extent, Sean Connery's Bond is the same character as Pierce Brosnan's or Daniel Craig's. To all of a sudden change him to a black man would be way too much of a cognitive dissonance.
The only way it might work is if Elba's character somehow managed to assume Bond's identity, which would make for an interesting movie in and of itself.