Just for your information, i already have all allied races unlocked, in fact i had all the reputations and questlines required before they released them.
Grinding world quests for a couple of weeks to raise up a reputation does not represent the level of trust with a faction. That's just a way to space out content over time, something that makes a lot of sense when that content is current, but stops making sense as a requirement for this kind of feature once the game has moved on to a new expansion. It still works for cosmetics since those are collectible goals, and a race is not, that's precisely why we are discussing this topic
The questlines? sure, those do represent how you earn their trust and are able to recruit them, the problem is that they are not consistent across the board. Some have massive campaigns, others have one zone, others a few quests and a couple of them have 0 connection with the quests required for them. I won't consider it a problem if those questlines were the only requirement, but the disparity between them doesn't make it a good solution either.
That's why i believe that as soon as the expansion is over, the only requirements should be having the apropiate level and faction to do the unlock quests.
Consider this: at level 109 you are exploring the broken islands looking for artifacts to defeat the legion. At level 110 you are on a boat sailing to Kul'tiras/Zandalar, the legion has already been defeated and we are at war with the other faction. It doesn't matter if you actually played the whole legion campaign or not, as soon as BfA was released the game assumed that any player at level 110 was part of those events, you can still go and do all the stuff if you want to, but you'll be treated as the hero who defeated the legion anyway. And that's exactly what needs to happen with allied races, in fact, it does happen in some way since we interact with NPC's from those legion allied races as part of your faction, no matter if you unlocked them or not, but unlocking them still has an arbitrary wall.