All MMO's have to move with the story and game mechanics of the game, it's an unfortunate aspect of the nature of an MMO but that's honestly the breaks. What Blizzard did was ambitious and unprecedented, with most likely other MMO's to follow in that particular path once the story or game mechanics of the game require it.
New players may not experience the old zones but that is probably for the best as the game mechanics of quests and phasing (Barely non-existent) needed the new zones, and updating the storyline and situation to be current (Cata-wise.)
Movement forward requires change and I praise Blizzard for having the forethought in streamlining the questing experience dramatically so that it's actually fun instead of a grinding slog.