People don't seem to understand what the hell people mean when they say the new models will be a selling point for the new expansion. There are many selling points for each new expansion. Dynamic shadows were a selling point for WOTLK. Water for CATA, and the new models could very well be for the next expansion.
These updates need a lot of developer resources to warrant focus on them over the usual amount of content we expect (which is now more than ever), and often these large updates and projects work better for massive projects like an expansion filled with new and large scale novel additions to the game because they can be justified in the place of getting new raid bosses or enemy mobs or whatever else a major content patch brings with it halfway in an expansion's life cycle. When you have a new raid, 5 new 5 mans, and a new continent, you won't tend to notice when they put a lot of effort into something like a graphical update at the same time. But like mogging, you never know, it could very well just pop up as a middle of the expansion content patch. I very much doubt it.
Further, if the new character models are a feature of the next expansion, they can justify not only updating the models, they can justify expanding and creating new options as well for customization, such as the ethnic lore based options Ghostcrawler confirmed they've considered in the past for adding to the game eventually.
---------- Post added 2013-03-16 at 12:03 PM ----------
It's no different than the major updates they added to the last 3 expansions you got for free like dynamic shadows and water. It's not just solely about getting people to buy the next expansion, they're going to do that anyways, that's when WOW sells best. While they would be a great incentive people have wanted in the game who probably quit a long time ago, it's a matter of justifying the amount of resources the project gets on it.
As something they can flash on the box for the next expansion, they can justify putting a lot more resources on the project than something they just slip in out of nowhere like transmogging. I think if they do not make this a feature of the next expansion, it would take even longer for the project to be finished actually if it's left as something like a side project they work in their free time without a concrete deadline.