With an increasing number of white actors out there screaming potentially good Bond; Dan Stevens, Tom Hardy, James Frecheville (who looks almost TOO MUCH like a Fleming/Bond) it's pretty much positive discrimination and calculated marketing aimed at an untapped ethnic audience to cast a black man who'll be too old when the time comes.
The character has no ethnic relevance or story to tell, so the approach would be to not really play on the color of the mans skin, but rather that the color doesn't matter, which makes you ask 'why cast a black Bond at all, then if color doesn't matter? Why shake the pot at all if you're not trying to prove a point?'
We literally JUST got back to Fleming's Bond with the Craig era, the way Bond is supposed to be with the likes of Casino Royale and it's not just critically successfully, it's MAJORLY financially successful as well so why take that big of a risk?
The answer doesn't come down to "Elba is a great actor. Period." It is marketing/finance departments doing research that may or may not suggest a potential increase in revenue by tapping into that marginalized ethnic audience, i.e. good old fashioned greed. Movie execs sure know how to kill a franchise rather by changing the characters in unnecessary ways or running it completely into the ground.
Here's the real issue; why the hell can't a character just be the character the way it was intended to be anymore? Why does everything suddenly need to be 'updated' or 'changed' when there is absolutely nothing wrong with them in the first place? Change just for the sake of change urks me off more than anything else in this 'debate'.