HAHaha thats crazy man, interesting take on it, cannot say I agree, I don't really, but its certainly an interesting point of view.People have been switching to the red team for nearly a decade. There was a mass exodus of raid guilds to Horde around MoP and there has been a drizzle to red team ever since. It's just that since last year, attrition of the Alliance population has finally reached critical levels.
The Horde is what is unique about WoW. Not other game lets you play as the warmongering savages, or as beastmen, or as undead who are crafting a plague to wipe out all life on Azeroth, or sinister elves who suck the lifeforce out of their victims. The Horde has all of the cool races. People who like Alliance aesthetics (ie, heroic humans, pretty elves, etc) have an easy time transitioning to other games, decreasing Alliance's population. People who like Horde aesthetics don't have an alternative game with similar aesthetics and are more likely to stick with WoW. WoW has been in decline for a decade so over the years you just wind up having more reds than blues.
It was only until the last few years when the snowball effect started kicking in. Standing in the middle of a ghost town Stormwind feels bad. People want to be on the side where they can hang out with other people. The average WoW player isn't a teenager with unlimited free time anymore. It's hard to progress on Alliance and you have to spend a lot of your precious free time waiting for groups to fill, if they even do. If you're red then you don't constantly get ganked in war mode. Even PvErs want to do war mode because it gives them an EXP buff. Playing Horde gives you an objectively better game experience. People who like Alliance aesthetics can enjoy the benefits of being on the red team while still playing a pretty human looking character by rolling Blood Elf.
Not much. You can allow players to queue and raid crossfaction, which will enable people who truly like the Alliance to switch back and still be able to effectively play, but I'd imagine that the remaining fandom at this point is overall Horde for the aforementioned reasons.