There'll be literally at least nine freaking months before expansion. The allied races will be probably available with preordering. Anyone wishing to play as a nightborne, void elf or whatever will have MONTHS to do the quests, THEN the prepatch, before we get into real BfA content.
If someone is crazy to be an allied race character but wants to wait until BfA release to come back to the game, then it's his/her choice.
(Plus, as a nice bonus, Insurrection gives you a mount)