By virtue of being playable, a race is conferred the status of being a major and important part of it's faction. Every race that makes up the Alliance has at one point or another carried a narrative that contributed to the overall plotline. Some races have done so to a lesser degree than others, but they still did it.
The unplayable Alliance High Elves have never done that and have merely served as a glorified prop thus far. A prop for the Blood Elves. A prop for the Kirin Tor. A prop for Jaina. And now a prop for Alleria and the Void Elves. But never by themselves.
The devs are not surprised by the desire. That some people wish to play Alliance High Elves is not a fact they have suddenly become aware of, they have known about it for many years. They simply have no wish to grant that desire because of the damage it would do to the divide between the factions and because Alliance High Elves are redundant as an option for a playable race, given that High Elves are already playable.
Actually I very easily can. I have several Alliance characters and leveled one as recently as last year. During my travels on my Alliance Hunter I found that I was predominantly dealing with Humans and to a lesser extent Night Elves and Dwarves. Draenei began to heavily predominate as I moved into Draenor and Legion content for obvious reasons. Gnomes and Worgen were also healthily represented.
The few High Elves I encountered stood out because I was specifically looking for this 'massive' High Elf presence within the Alliance. I encountered a few, but mostly related to the Silver Covenant as i expected.
All playable races within the Alliance, the real Alliance, were adequately represented.
Undead Elves.
If we keep it to the present then Alliance High Elves have been ruled out without any possibility of being added. You can't limit the discussion to the present without conceding the discussion entirely.
Your prospects in the future are almost as bleak but we have to entertain they hypothetical if exceptionally unlikely maybe. After all, we have Blizzard's rationale for not adding playable High Elves. They would be basically identical to Blood Elves. All attempts at reimagining them have just been attempts at reinventing the wheel, they don't go far enough whereas Void Elves did go far enough.
So either Alliance High Elves will be the equivalent of reinventing the wheel so that it isn't round or Blizzard has to be convinced that the faction wall isn't important anymore.
Given the former is impossible, you must aim for the later and hope to convince Blizzard to dismantle the two most iconic factions in videogaming, which is the consequence of the removal of the faction wall. Good luck with that one.