You are making the mistake so many people do on this topic.
The group who calls themselves “High Elves” on Alliance are being asked for, not the race, “High Elves”.
Blood Elves are a race of High Elves sure, so are Void Elves.
But neither are the group that calls themselves “High Elves” to this day in WoW. Both Blood Elves and Void Elves identify differently with each other and themselves.
So do the ones who call themselves “High Elves” and have stayed with the Alliance, even when the majority of their people seceded from the Alliance and later came on to identify themselves as Blood Elves.
Again, talking about your ancestry as missing the entire point of the discussion. This isn’t about races, this is about a specific group on Alliance that fans of wanting to become playable.
They have said it is possible that High Elf customization can come to Void Elves, I believe there are threads on this subject. Therefore, Blizzard has clearly indicated it’s possible for the “High Elf fantasy” to exist on Alliance.
EDIT: To clarify it a bit more. When asking for “High Elves on Alliance” people (for the most part from what I’ve seen) are not trying to say “The race high elves should be on the Alliance faction” as here I can see why those who play Blood Elves can be upset.
For the most part, players are simply asking “this group of elves that call themselves ‘High Elves’ on Alliance should become playable”. I’m not sure how with this clarification someone can argue that those asking for High Elves are asking for a race.
If this were the case, then many High Elf fans would be satisfied with the Void Elves as they are also a race of High Elves.
That Blizzard has said High Elf customization can come to Void Elves, also shows that Blizzard themselves don’t see Void Elves as High Elves for Alliance.