If the old style sells enough, I think they'll absolutely keep it around. In fact, I'd expect them to back off on the 'transition' and rename the new-style MBP an Air.

After all -- despite the mini, nano and shuffle -- the iPod classic stubbornly remains. Even after all these years. Not to mention the 13" MBP which seems (without the benefit of sales data) to be fairly pointless.

Apple won't hesitate to risk cannibalizing itself with new products. But they don't have any history of forcing the issue, if the old products still sell.

I think that if they can, they will. "Can" being dictated by the market of course. Look at the Mac Pro - arguably the most ignored product line Apple sells, yet it still sells. If people continue buying the non-Retina MBPs, I have no doubt Apple will continue updating it, even if slowly.