They should never fully scale the zones from 1-100 that would make no sense at all.
Do it in brackets, a group of zones for 1-20, 20-40 & 40-60. Outland 60-70, Northrend 70-80 and so on for the respective xpacs. And rotate world quests in them every week (or day) at max level, like one week for example Kalimdor WQ's or Northrend WQ's so you have a reason to go back out into the old world.