I think there are some marketing concerns here:
1) MMO and computer games are eating into D&D's traditional audience. This is evinced by WotC's own "attack ad" on MMOs and evidence that, while anecdotal, is extremely broad and consistent in reports.
2) D&D's design anticipated this problem to an extent and utilized many of the features of computer games.
3) #2 led to something I call the media utility problem. It works like this.
A: When two similar media have the same audience, the temptation is to make the less popular form emulate the more popular form.
B: However, this only begs the question of why the consumer doesn't just choose the more popular form, once the distinguishing characteristics drop to a level that makes the transition easy.
Keep in mind I'm talking about consumers, not users. RPG consumers are less numerous than RPG users. A large player networ does not translate into people who want to actually buy stuff. In the case of D&D there were people who never made the transitions across editions for 2e, much less 3e.
MMO users are, on the other hand, *always* MMO consumers. The nature of the MMO hobby makes it so.
3) Therefore, D&D needs some kind of distinctive touchstone that makes it *different* from the leading media forms of the fantasy genre. It can't be like books or computer games. So from a marketing standpoint, miniatures do nothing but add to D&D's value. Furthermore, ater the disaster that was Chainmail (which basically begged the question, "Why not play Warhammer?"), WotC now has a successful brand in a miniatures niche that does not sufer from the media utility problem.
4) It's consistently been WotC's policy not to dilute a brand's identity. That's why there's no MtG/D&D crossover. It would be the company's tendency to keep a strong connection between the miniatures and RPG sides of the brand.
5) Would this mean a big change for the miniatures game? Not necessarily. It would mean a change in the way the D&D RPG is presented. Remember that D&D is not even marketed as a "roleplaying game" to consumers outside of dedicated hobbyists. It's almost always called an "adventure game." There has always been a desire to emphasize that D&D is not a part of the community of RPG brands.
So the meaning of the D&D brand is pretty flexible, going from straight-up miniatures and no RPG to heavy RPG elements. The question is how broad D&D's identity will remain, and what combination of elements will be presented as the core experience of the game. I suspect that the RPG might be a set of rules on top of the miniatures systems, but this kind of transitional game has failed before (GW's Inquisitor), and anybody designing the new game will know that.