What retcon? We already know for a fact that there were absolutely worgen who were part human and not 100% pure beast prior to cataclysm, hell, even during classic. And while Blizzard can absolutely modify the lore, they would be turning their backs on YEARS, not just one occurrence (I.E. the draenei), YEARS of naga being one of the most hated, antagonistic races in Azeroth. The naga have done something to piss off basically every race within the Alliance and Horde. If naga are introduced as a playable race, it would be proof that Blizzard no longer cared about the story of Warcraft.
They have larger populations than the high elves. The draenei, goblins, and trolls have towns and cities to their names, which is proof of this. Gnomes may only have Tinker Town, a similar predicament to the high elves, but they are also a solid part of the Alliance military and obviously have a large enough population to wage war and be spread across the world. There's a reason we only see high elves in "large" numbers in or around Dalaran. The high elves' biggest "settlement" was lost to them when the quel'dorei there became wretched.
Give me a reason why Naga should join the Horde when they A) slaughtered or enslaved the majority of the force that was sent to Vashj'ir B) Drove the Darkspear away from their original home and came back to kill them off C) Had numerous lesser conflicts with the Horde D) Can arguably be partially to blame for the madness of Kael'thas, as he followed them, and through those events, was ushered into Outland (which was Lady Vashj's idea), where he got a taste of demonic power E) are basically all servants of the Old Gods. Blizzard can't possibly ignore all of that.