That idea's definitely been batted around before, and anyone leveling an alt can definitely see the benefits. But think of what you're giving up with a new rule like that: Deadmines, Wailing Caverns, Scarlet Monastery. All of the epic quests and reputations, all of the great old-world vistas and settings. Plus, new players to the game will find the old world even emptier than it already is -- everyone who has a high level character will already be leveling through Outland or Northrend.
Do we really want to abandon the old world for good? Blizzard doesn't think so -- Palehoof unofficially suggests that Blizzard is happy with the subscription fees for the two weeks it takes you to level your character, and Bornakk officially says that Blizzard is happy with the way things work now, and if they see a problem, they'll fix it. You have to think that they'll eventually allow this kind of "powerleveling" in some way -- as we move closer to level 100, it'll just be silly bothering with the lower levels considering how far ahead the new content is. But for now, you'll have to stick to leveling the old-fashioned way, because vanilla WoW isn't going anywhere.