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Sooner or later, it's likely that all non-Retina Macs will go away. Probably not this year, though. The screen is hardly the only differentiator between the Air and Pro; current 13" Pros have a CPU of twice the frequency of that found in the current Air, and far faster graphics, while the 15" Pro has quad cores, faster again graphics, and an option for discrete graphics.
There will not be any more non-Retina displays from Apple in the near future (2 years max), they're just waiting for the fabs to catch up and for the costs to drop. This is one of the ways to do this, by reusing the iPad's screen technologies to adopt it for the Airs. This makes all the sense in the world to do, especially if Apple wants to keep it at 999$ priceline.
Apple will be seem as "falling behind" if they do not get rid of their non-retina displays sooner as the rest of the industry starts to catch up. Especially with Sharp's IGZO panels being used in many new products being released this year. Dell, HP, Lenovo, etc are all going to be releasing 4K displays in addition to including retina-class displays for their tablets and laptops.
If they are both Retina, then the weight difference is the biggest deal. Reducing the screen size from 13" to 12" would let them make it significantly lighter than the MacBook Pro.
Right. And people will ask themselves ... why have I paid more for a heavier notebook?
I imagine that a 12" retina MBA will mean the death of the 13" MBP. I further imagine that Apple is perfectly OK with that.
I'm really struggling to imagine customers are going to be ok with that.
Just because a smallish form factor proved propular doesn't mean you're not suddenly going to find a size which is a dealbreaker.
12" is tiny and doesn't really improve portability for a laptop-like device (which goes in a backpack).