You don't dump your bestselling product.
The 13 inch Macbook Pro is far and away Apple's bestselling laptop, so they won't get rid of it. It's a very nice machine, but I am not quite sure why it is their bestselling laptop, given that the 13 inch Air has the same graphics, an only somewhat slower CPU (1.7GHz rather than 2.4GHz, but substituting an SSD for a Hard drive makes up for a lot of this), a higher resolution screen, is much lighter and more portable, and costs only slightly more for the base configuration. (If you order the 13 inch Pro with a 128Gb SSD like the Air does, it actually costs more).
Although, given how many times I hear people in Apple stores ask "Why does that one only have a 128Gb Hard Drive, when that one has 500Gb?" perhaps I do. Non-technical customers seem to have "How big as the hard drive?" as one of their key questions, along with "How many megahertz does it have?", so perhaps that is why they buy the Pro over the Air. They seem to have no interest at all in screen resolution, though, which I find a bit baffling. (I have the 13 inch Air, myself).
There actually is going to be a bigger technical difference between the next Air and the next 13 inch Pro, however. Unlike the 15 and 17 inch Macbook Pros, the present 13 inch only has a dual core CPU, due to the power and heat envelope for the present enclosure requiring a CPU with a thermal design point of no more than 35W. Quad core CPUS with a 35W TDP don't exist for Sandy Bridge, but they do for Ivy Bridge, so the next 13 inch Macbook Pro will be Quad Core (at least in some configurations), whereas the next 13 inch Air will still be dual core.
The next 13 inch Macbook Pro may well lack an Optical Drive and may well be thinner than the present model, but it will still be a separate model from the Macbook Air.