That kinda has been the story they have been telling since Burning Crusade, the rise of the Blood Elves was the first step. And I do agree that doing the story of it right would be important, and good fun. Last of a dying race is a fantasy trope that can be used to tell some very powerful stories.
As for using the new human character models as an excuse, oh come off it. I've been inactive for a while, so I can't post links, but a little googling will do the trick for you.
About 70000 years ago humanity almost died out due to a genetic bottleneck, something called The Toba eruption. There were so few of us left that we would from all intents and purposes have succumbed to inbreeding over a few generations, High Elves seems to have hit that level of low. The difference between them and Void Elves is that the VE's can create new members of their race not just through birth, but also through "void corruption", that's a big difference. Then, of course, the counter-argument is HEs can change the minds of their BE counterparts and they can defect to the Alliance, so it's kinda circular, I get that, but when BEs defect it seems to be because they join the VEs.