That is true, but there is somewhat of a problem if the vast majority of your game is trivial.
Maybe not all zones should be level scaled, or at least at the same time, but having a good reason to return to the barrens is just as important I would argue as being able to go back and feel like you are more powerful.
Zones having a max level that isn't level capped is a solution. I .E Max level for mobs in Redridge isn't 60.