Originally Posted by
Grocalis
Sometimes they update the old world to represent changes in future expansions if the change is cosmetic, sometimes they can't. This is one of those times they can't. Why? Because Dalaran is a HUB CITY. It has banks, inns, merchants that sell gear for T8, 9, and 10. It has the Tuskarr for the Northrend Fishing Tournement, Daily Quests for Cooking Awards, Daily Fishing Quests, the entrance to a old dungeon, unique mounts and pet merchants, etc...
Imagine if you are playing Horde, and you do the Purge of Dalaran storyline. You decide, "Hey maybe I want that Mammoth mount." You head to the portal room in the Vale only to find the Dalaran Portal now missing. You ask a few mages to portal you, but most can't because they also did the questline. You find a low level mage willing to do it, but now you are in Dalaran with all the NPCs hostile and ready to attack you. Good luck getting that Mammoth.
That is why it will never be phased into an Alliance town.