Its quite possible given the capability of mobile GPU's. Also, its possible to offer the higher resolution screen at a higher price. This is the SAME thing that Apple has been doing for years. Remember the 17 MacBook Pro one came with a standard res and higher res screen at a higher price point? How about today, the 15 MacBook Pro comes in a standard resolution screen and a more expensive higher resolution screen.
Only those ignorant of Apples offering in the past would not see the possibility for offering two screen resolutions.
Although I don't think they will... you are right... they could offer an upgrade to a retina display? But from what I read, they just aren't making them that big now.
If they did... that would be cool... I could see some people willing to pay more for a kick butt screen.