This is not needed at all. First, if you want to finish a zone, you can do it. Nobody prevents you from it just because your quests grey out. You just have to be clear about your priorities.
Second, I don't know how this would work out with talents and abilities. Currently, I have the feeling that every time an expansion is released and they rework the spells / abilities and talents of classes, they automatically destroy the balance of all level brackets before. Like frost mages did not had an aoe spell up to lvl50 or something, because blizzard only then comes into play (when they did the excessive pruning of spells).
Also, I rather want to fly through levels where my ability toolkit is limited to 1-3 buttons ASAP, so I can finally play something which at least somehow resembles a rotation or priority system. I would not want to be hampered with irrelevant scaling in this regard. And since I seldom get all items I want to collect from raids and dungeons, I love the option to come back later and being able to solo these to complete my collection (mounts, transmog, pets). Having to get a full group just to run transmog content would be contraproductive.
Finally, if things scale up to your level, the rewards from mobs and bosses should remain competitive. This can possibly be done for quest rewards and boss loot (but it would be quite an amount of work to scale all these items), but what about profession items? You cannot craft anything useful besides bags with BC cloth or leather if you are questing in WOTLK or Cata areas. Professions are already quite useless while leveling, at least when you have enough heirlooms, but in a scaling world, how would profession items be handled?