If Blizzard wanted to do something different, they could easily just put San’layn on the Alliance because they feel like it. Just like how they made up void elves on the spot, pulled Lightforged Dranei out of thin air, made the Blood Elves go Horde in the first place, and oops, Nightborne - Night Elves - are Horde too.
GD posters that do nothing but tell people who want a playable subrace wrong all day don’t get to decide what is and isn’t. Blizzard has already decided the San’layn are not going Horde. If you’re too lazy to do that quest, you 100% do not get to say anything about the lore on that. If it’s gatekeeping over aesthetics, oops, Forsaken exist on the Horde. Do we really need double of the same “undead” concept on one faction, when the Horde already got two magic elf races with similar aesthetics?
Worgen also caused people to whine because “it doesn’t suit the Alliance”, but they’re Alliance, and it works. Vampires (even as a subrace) and Werewolves being on the same faction would actually be that “same but different” that Blizzard tends to do with their world-building as well. Alliance has a heavy “old school fantasy” vibe to it that suits vampires.
The san’layn aesthetic kit would do more to expand void elves into actual monsterous elves - mutated elves, the thing a ton of posters who derail these topics all day throw temper-tantrums about, than it would to expand blood elves into anything related to their actual story with the sunwell.
With Void Elves and Dark Irons on the Alliance, it’s anything goes, and it’s going more towards them not being in the Horde because oops, they died trying to join, and the few that were about to make it turned on the Forsaken.
I’m not even going to bother going on about the lore behind how void and undeath counter each other, so they could actually aid each other in finding some purpose in their existence. The Void Elves could aid straggling San’layn - another race with a low population - more than Quel’thalas ever could, or would want to.
If Blood elves want additional customization, they can have the ones their race is already themed to have and has in the lore; fel and light. Fel and Sunwell aesthetics.
You can’t gate keep something that you don’t actually have in the lore, unlike high elves, which the Alliance does canonically have.