Ok here is something I don't get about your proposal. What is the major harm in allowing Alliance to play the already existing High Elves in the Alliance?
You say have "new elf toons". None of those new elves are going to have the history as in, since WoW's inception to today history, that the current High Elves in Alliance have. Your proposal negates that when that's one of the major reasons in asking for High Elves. This is why it's bad.
Also it is a retcon because it's never been shown that there are High Elves who regret not going with Lor'themar because every High Elf shown has been for the Alliance or gone off to do their own thing. But like Shadow of the Sun shone, even those High Elves not part of the Alliance don't want to go back to Silvermoon now that they're Horde.
There is clear evidence High Elves = either Alliance or independent, and hateful of the Horde. So it becomes a retcon.
Again, if this was just about visuals, then "Blue Eye Blood Elves" aka Void Elves would've stopped the High Elf request, not bolster it. The High Elf requests today have been nowhere near as organized pre-Void Elves.
That should be sending a clear message to Blizzard.
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The majority of posts online that shit on WoW having too many elves are the real minority because if that were the case then Blood Elves wouldn't be majorly popular, and Void Elves wouldn't also be so high in popularity as race choices go.
The majority of people love elves, and despite the 4 in-game, still want at least 2 more. Just like how there's 2 trolls in the game and people still want a 3rd. Same for Dwarves.
People wanna play the characters they find attractive and interesting, and in modern times elves always will be some semblance of attractiveness as one of their themes.
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Imo, I think the real chance of High Elves happening is what sets this particular race to have the most pushback. It's super dumb tho, if we were talk about it at its most superficial level. Blood Elves aren't going to stay as the only fair-skinned Elf in the game, because clearly we see Blizzard isn't adverse to adding 3 different purple elves into the game.