I think we are basically talking here about the successor to the MacBook Air. The origin of the MBA was as a premium product, and I bought mine back in 2010 for using it with professional software (when most of the people were telling me it was a wrong choice and that I should have chosen a MBP instead... but my decision proved right, the MBA was perfect to run professional software in a very light MacBook, yet with a decent screen size, and it was indeed the pioneering concept of the MBPs we have today).
Nowadays the MBA is left as a low-cost entry solution, but its concept (ultralight with decent screen size) has no successor. People will argue that the 13'' MBP has the same weight as the MBA, but, however, the 13'' MBP is not the MBA successor (if it were, it would be lighter than the MBA rather than having the same weight, because we are 10 years after the MBA... weight would improve in the successor, rather than remaining constant).
Regarding the pricing for a 14/15 inch rMB, I think it would be quite similar to the 13'' MBP, and it would make sense: given a price, you could choose between more performance with an smaller screen and heavier weight, or less performance with a larger screen and lighter weight. It would make sense.
BTW, I don't really think the current MacBook pricing avoids "cannibalization", because the whole range (except perhaps the MBA) falls within less than $1000 difference. If you can afford the 12'' rMB, you can also afford the 13'' MBP with just a minor extra cost effort. I don't think the choice between the 12'' rMB and the 13'' MBP today is a matter of money, but tastes/preferences.