It's not going to happen. It would steal sales away from the MacBook Pro, and the cost would be minimal between the two.
IMO, it's by no means out of the question. After all, even the current 13.3" MacBooks will yet go through a number of significant upgrades in future. At some point, it's likely they'll have dedicated graphics. They just won't be on a par with the MBP's graphics (& other extras!).
So it's quite feasible, & it needn't affect MBP sales. Overall, it could also significantly increase the number of switchers from PCs!