If the Heartless got to Mickey in the past, then their would be no current King Mickey. If there is no King Mickey, then Xehanort's sucess rate goes up higher than before because there would be less obsticals in his way. Of course that world's timeline changing would have affect the whole series. Although it wasn't mentioned afterwards, that door was still there. Therefore, it provides a passage to the past. Not only that, but Pete also managed to conjor a door to the past and entered it with his Heartless cronies, which is what caused the Cornerstone of Light to be infected in the first place. Someone could conjor up yet another door if they knew how, or even if it was by accident.