I believe that, 'Retina-class' displays allows Apple UI designers to optimize for the physical size of displays, NOT their resolution. Because UI is first and formost a human issue.
Therefore, if we are to believe that all Apple screens will be eventually 'Retina-class', we also have to take into account that any and all physical screen sizes are dictated as being optimal for human context, not whatever happens to be in supply. For instance, if the 9.7" screen size is ideal for two-handed tablet use (the iPad), then I take it to mean that it will remain that way no matter what the resolution.
SO, it makes more sense for Apple to design a 7" device because they intend for a new product category that encompasses an ideal set of human uses, not just because it conveniently can scale down an older product.
My point about resolution independence is that all iOS products essentially run the same OS. And wouldn't it be nice if iOS was resolution independent just like Mountain Lion. It would make developing for the platform a little bit easier, and users would benefit as well. Designing apps would still be difficult, but I believe the applications themselves would be more resilient wrt to UI. I also believe that it gives Apple more flexibility when they do happen to change their physical screen sizes/ratios. If Apple were to suddenly decide for some reason to make a 14" 16:9 tablet at some ungodly resolution, it would be great if iOS was resolution independent, would it not?