If they did this, there wouldn't be much differentiation from the rMBP other than the retina screen, so Apple would cannibalize its own rMBP sales which in *your* scenario would have a HUGE margin of profit over the cMBP because the retina screen alone would be a ~$400 profit. Nobody would justify the 50% price jump to the rMBP if the internals of the cMBP are almost the same.
Not much different than when the 13" rMBP was first released, and at least now the 2015 13"rMBP would have battery life and faster SSDs. The fact that the cMBP is still around shows there's still plenty of market for such a machine like the cMBP, regardless of how much Apple wants to force super thin MacBooks and iPad Pros on the user base.
My main problem with the cMBP is the fact that they are charging $1100 for a 4 year old processor, 4GB of RAM, a 500GB 5400-rpm HDD, and a 1280x800 TN screen in 2016. Not so much the concept of the cMBP itself.