Back in WOTLK development there was this idea that you would have to sacrifice one of your max level characters to become a Death Knight. It never panned out, because logistically the idea of class-swapping a character was not feasible. It's still not feasible, you can't pay for a class change even if you wanted to.
Covenant ARs are basically inevitable at this point, and there could be an opportunity to pull off this character sacrifice system once again without the problem of classes. Instead of creating a new Covenant race from character select, where they'd be low level and have to journey through quest content, you sacrifice a max level character to become a Kyrian or Sylvar if you're Alliance, Maldraxxi or Venthyr if you're Horde. There is simply no option to create a fresh Shadowlands race character.
Functionally it is just regular race change, facilitated through some quests instead of $25 off your credit card. Unlike class changes, race changes are easy peasy. Financially the character-sacrifice option is a better deal for players. It would be a free, one-way, one time deal for each max level character. You'd only have to pay to race-change back into something else.
Lorewise, there would be no need to explain why the Covenants are coming to Azeroth, let alone join a faction, or collect bear asses. This is obviously the biggest glaring problem with the idea of Shadowlands Allied Races, why the hell would they migrate from their realm to ours, or join the Alliance or Horde, it really makes no sense. Character-sacrifice method sidesteps the big lore problems completely. It's the same character, Choom the Tauren Death Knight simply becomes Choom the Maldraxxi Death Knight. Since all Shadowlands ARs are created from pre-existing characters, they don't have to quest or level up (until the next expansion), which would be a weird thing for a being from the afterlife to have to do.
Post-Shadowlands the Kyrians do not mind-wipe new Kyrians. So your Human Paladin turned Kyrian can just go right back to what they were doing before. No explanation other than 'players have agency' is needed to explain why they'd commit themselves to Azeroth's defense instead of performing their duty to the afterlife.
The two points, cost and lore, are compelling reasons to implement Shadowlands Allied Races by means of sacrificing an existing max level character.
Thanks for coming to my TED talk