They should remove the restrictions a few patches into BfA in my opinion. No point in having them locked behind old content when said races advance in the current content's story. I think of it the same way I think of the Draenor Pathfinder achievement; restricting things behind having to slowly go through outdated content provides the game with nothing else other than people never actually experiencing the content.
If World of Warcraft was still the game it used to be numerous expansions in the past, where catch-up mechanics weren't a thing, then I'd wholeheartedly agree, but right now I see the removal of restrictions behind allied races and the pathfinder achievement no different than, say, AP catch-up mechanics or the removal of gating. If there's things that allow players to get something for less effort, then there should be no exclusivity on things like allied races either.