Before Cataclysm, people were gushing about how great the updated classic zones would be and how it was about time flying was allowed in the old world. Literally within a week of the expansion's launch the same people were acting like it had always been that way and looking for something new. Nowadays the expansion gets lambasted for the fact that endgame, current content was sacrificed to gussy up the Vanilla zones.
New models on old races are the sort of things that people rabidly look forward to and promptly forget all about once they're in. You don't get any gameplay out of a new character model. You can't really admire it when you're putting gear of varying graphical quality all over it and actually, you know, playing the game. It's just there.
This isn't to say that there aren't merits to redoing the models, or that they shouldn't redo them. There are, and they are. But there's no way in hell they're going to try to pretend it's the same thing as adding a new race or class when the time comes. It just plain is not a selling feature. The fact that a game made after 2004 looks like a game made after 2004 isn't a feature, it's an expectation.