In my view, the best argument for Helves is the Nightborne. If the Horde can get Night Elves of merely a different political faction, I don't see why the same can't happen with High Elves. Just make the model slightly different as Blizzard did with the Nightborne. Hell, they're still close enough that it's hard to tell the difference depending on their gear.
Nightborne are just a different political faction of Night Elves that got shoehorned into the Horde. I wouldn't see any difference between that and High Elves.
I mean, personally, myself, I think we have more than enough Elves as it is. Too many in fact. But it's disingenuous to see Hordelets hiding behind the same reasoning when that means they shouldn't have been given Nightborne at all.