Tbf, only horde elf fans consider that "shitting" on the horde, but I hear your point. I don't think what happened in TBC needs to be replicated on the scale that it did, I think the alliance is still well loved, it's just rusty, it will take a little fire to light it and it can run..
The blood elves were a catalyst back in TBC, but it was much more than them, every narrative decision put the horde at the centre, to raise its profile, every cool development favoured the horde and if it was alliance centric, it was shared with the horde, the horde either had to have the same or better every single time, the net result was the shifting perception of the horde from the lame, evil faction to the cool faction. Cultural shifts also influenced that. Off the back of the 80s/90s your noble virtuous hero was the most popular, as the 00s progressed your edgy emo , anti-hero part monster dude became quite the in thing.
With the horde's reputation much better, it is much popular , it doesn't need high elven assets in the blood elves to maintain that. This is why I know it can afford to lose those assets even though the race is still playable on the horde. The alliance on the other hand would HUGELY benefit from high elves making a big re-entry splash to the playable spectrum and the kaldorei actually truly shining in all the aspects of their lore on the alliance this time.
The reason why the horde needs to lose Silvermoon/Quel'thalas and Suramar is to widen the dichotomy of the two factions , distinguish them better, and heal the identity crisis on the horde, which exists precisely because the blood elves are far too high elven an d have a too high elven and thus alliance type of civilization /set of assets etc, same with the Nightborne. That's why you're removing them, the alliance popularity doesn't need the blood elves to lose the Silvermoon or the Nightborne to lose Suramar - the horde needs this more. however the alliance gaining it will make the alliance shine brighter - still alternatives could be simply building an even better pair of cities for the night elves and high elves - so the blood elves and Nightborne don't lose anything.
The problem is if the blood levs and Nightborne don't loose those high elf things and night elf civilization things, the horde still feels too alliancey, too much alliance on it, because those things are too alliance themed. this is why they have to go.
If you have followed my posts throughout the years I have suggested many things, never this, I've always bene in favour of not nerfing anyone and instead just buffing where needs to be, but sometimes, a little pain and loss can go a long way and open the door for something much better, and shouldn't be avoided - when mistakes need to be corrected.
I cover this extensively in my earlier discussion, I thoroughly recommend reading it. The key to solving the problem with the alliance numbers in understanding why it is like it is.
Many people think it's things like racials, or the blood elf model , or horde guilds. It isn't. It's popularity and perception - it was the lore/narrative and presentation changes that made the horde very cool, that changed it's perception.
You need to make a cool shiny big song and dance over the alliance for a little while, and favour it for cool and nice things - this will do the trick. you don't even need new stuff, because the alliance races actually have a lot of cool things about them totally underutilised , ignored and siphoned off to the horde. Quel'thalas, Silvermoon and the kaldorei civilization are examples of things siphoned off, lame dwarves, lame night elves that have little magic, no well of eternity use, no fancy world tree magic use etc, going from a great multi-spectrum race to effectively a niche forest elf only expression - boring dwarves, Draenei who are just always agreeable - it lacks fire and imagination and grossly under utilised. Night elves have fascinating things in their lore, Suramar is evidence of it, and that's not even all, the Illidari are under utilised, the whole mystic of the stars - they call themselves children of the stars, had astrology towers in their civilization and so much mystery to the order of Elune that was intertwined with the mages and the people and the druids - so much non used, only the same one dimension druid take for most of it..
Want to see dwarves interesting/ you watched the hobbit? you played Warhammer? Dwarves can be so much cooler than they are making them in Warcraft.. it's all perception hun.
Do the horde need the blood elves? Well look at classic relaunch, there are no blood elves in the 2018 relaunch, but the overall population is 50/50 now why is that? Because the horde is no longer perceived as the lame faction. it didn't even need eye candy models, those were just the incentive back in the TBC days to lure players over and establish a trend...once the trend is established it is no longer need, emphasis should have switched to the orcs and trolls, they should have done majestic troll and orc places like Daza'ralor/Zuldazar rather than sling more alliance elven stuff (like Suramar) over tot he horde. Void elves should have been darker and a direction tied into blood elves while high elves the vanilla version gone to the alliance. But that didn't happen.
So what do we do now to change alliance perception.. make the elves incredible like they can really be, for being alliance type elves, in their native way - don't shunt that to the horde, the horde is not the faction of high elves traditionally, just because blood elves have sat on the horde since TBC, they've never become more horde like, they've remained very high elven which is an alliance template - if the alliance popularity is low, because it's unattractive and uninteresting, make it exciting. Find out what makes the alliance exiting and the players excited about it and make the shine bright and glorious like the sun.
And if you have any sense, you'd quickly see it is high elves and Night elves that do more for the alliance than any other factions. Even not playable, alliance players wish for and make more posts about high elves than they do dwarves, Draenei, worgen, gnomes, pandas COMBIND... it is alliance fans that talk more about Nightborne lore than horde fans do .. horde fans only talk about blood elves (Nightborne talks are just moans about the models), Night elves are the other massive alliance topic - there is such a desire for them to be cool, powerful, fierce, badass, magical etc - it shows you the things the alliance players like and want are not been given them, if you want all those players who've transferred to the horde to go back.. make their alliance cool again, change the perception.
This is why I feel it needs a big splash again.