You only think that because you don't like the suggestions, but I am not heavily biased against the horde at all. I'm not even biased against the horde elf section.
But you are so convinced you understand what I am, you haven't even considered it properly.
That elven identity in the blood elves and Nightborne is alliance. It's too alliance, it it is left on the horde, you have the horde remaining with alliance on it (is that really the best thing for Warcraft? Alliance on the horde too?). Technically speaking in the narrative sense, taking those Blood elves back to the alliance as High elves and those Nightborne to the alliance by turning them into night elves is not removing their elven identity at all but placing it on the faction that represents such things.
The remnant/few remaining blood elves and Nightborne, do not have to be elven in that high elf sense or night elf sense to be considered blood elves or Nightborne. It is an assumption of yours that in order to be elven, they must continue in the way they are right now. The way they are right now is too alliance based, so they simply need to change that to make the factions more distinct again. AS for losing Suramar and Quel'thalas and the culture stuff, it removes or at least significantly reduces the alliance lure that they have been so far. High elves fully being available on the alliance in all their glory at the same time as horde blood elves losing the high elven assets and behaviour will squarely put that version of elvenness on the alliance only. So players who like that and want that - that is alliance for you, they go there.
This is what you want in this game, and what it needs, the alliance is about all the things the high elf kingdom and the night elven civilization represent, the humans and dwarves too - that type of civilization, type of ideals, nobility, arcane magic, way of thinking etc, these are the hallmarks of what the alliance is built upon. it is the faction that represents the ancient cultures of humans, elves and dwarves on the Eastern kingdom that vein( derived originally from the kaldorei - the first of that type of living).
To have that on the horde, and keep it on the horde greatly affects the alliance and dilutes the horde. you may not like this conclusion and my insult me all you want. But as long as blood elves are parading as high elves, and Nightborne living high as kaldorei civilization elves, while their alliance counterparts are crappy remnants, and the alliance races clearly second best in all those things the alliance is known for, you would always have a net flow to the horde, because the horde will continually lure alliance minded types because of blood elves being this way and Nightborne in addition to those drawn to the horde faction for the original horde races they are.
If you want to repopulate the alliance, elves have to be more attractive on it, more visible. Do this well and the alliance will be an exciting place for those type of players and will attract enough of its crowd back. And since blood elves and Nightborne won't be losing their models, the horde will not fall apart either, their elves can afford to suffer a little (I'm not saying this cos I hate the horde) and can afford to be redrawn into something else - because they are on the horde faction, and being something different from high elf and Night elf is better suited for the horde because those races aren't horde based.
It's not the only way, it has never been, but it is going to be the most effective way and the fastest, and can be the greatest way if done properly. There is a reason I suggested this is done by the horde elven npc populations turning back to the alliance instead of being destroyed. there is a reason I suggested that Silvermoon/Quel'thalas and Suramar all go to the alliance. There is a reason I suggested that more troll, orc, tauren, goblin assets and cities come into replace the elven stuff lost - in a visible way because it promotes the horde core fantasy . There is a reason i also suggested that a little later, the blood elves and Nightborne build up again, but this time not as high elves or kaldorei types, but something more unique - this isn't hard to imagine, I gave examples of the naga, the Pandaren, the San'layn, the Venthyr as examples of what unique can be, and I gave examples of Kael'thas' TBC Sunsworn and Elisande's loyalists as examples of how these elves were actually shaping out to look quite different from the high elf/night elf model they were instead given - as an example of how it could go, not insisting his is how they should go.
If you want your white shining knight, really, the alliance is where you should be, not vicariously living alliance dreams on the horde through noble Nightborne and white knight blood elves - that's an alliance fantasy. if you take that fantasy to the horde too, and make it even more attractive like they did, you make the horde more attractive over the alliance to alliance type players, and you diminish or pollute the vein of the horde.
I'm not being angry or vindictive or horde hating at all by using this language, a person that wants to restore the core and distinct fantasies of both factions doesn't hate them. If you perceive the loss of high elven and night elven facets to the horde elves as horde hating, then you have an elf and alliance centric view on the horde. This is problematic. You think a person that wants to remove the alliance fantasy themes and elements on the horde is spouting horde hate when the only ones negatively affected are the two most alliance horde races who in his proposal actually, eventually make new gains that could even equal or exceed their previous state - that is not horde hate. But you probably never read that far, you read a few lines, jumped to conclusions, and tried to sound intelligent about such an unwise act - feeling that would be enough... Well, it isn't, all anyone has to do is simply read what has been said through and it is quite easy to see.