Like many of the posters here, I believe they will phase out the current design in favor of the Retina MBP. However, I think it will take some time before they get rid of the regular Unibody form factor.
-SSDs have gone down in price, but they still aren't "affordable" for the average consumer. Within the next year, it seems that they will become a much better deal.
-The Retina display can't be cheap. When the price of that comes down, Apple will be able to incorporate it into the 13 inch MBP, and completely transition away from the current design.
-Some members will disagree with me on this point, but I don't think the general public is ready to completely move past Optical drives. I think the Retina MBP of today is going to help get people used to the idea of a full-fledge notebook without the ODD.
If Apple is able to sell Retina MBPs at around the current price of Unibody MBPs, they definitely will eliminate the previous line and solely sell Retinas.