I don't know.
The 13" MacBook Pro, both retina and non-retina, seem to be in a precarious place to me, considering there is the 13" MacBook Air. The base price point of $1699 for the 13" rMBP just doesn't seem right when you can get a fully-loaded top of the line 13" MBA now for $1699, and that gets you a 2GHz i7 Ivybridge, 8GB ram, 256GB SSD, a 1440x900 resolution, and a card reader to boot. Those specs are pretty damn good for a machine of that size, and for what most people do with a 13" machine.
If anything, Apple needs to consolidate its 13" category. The non-retina 13" and 15" will most like EOL in the next 1-2 years, but at that point, what would really define the 13" MBA and 13" rMBP other than size and spec bumps? Perhaps then, Apple will combine both and call it the Air. Or perhaps they'll discontinue the 13" Air and leave us with an 11" MacBook Air, 13" MacBook, and 15" MacBook Pro (creating distinct product lines again).
I agree that this makes sense. Perhaps there won't be a need for three lines, though. If all of the laptops are thin, have Retina displays, have no optical drives, have no hard drives (just SSDs), have no user-upgradable parts, and look essentially the same in terms of design, why not just have one product line, such as "the new MacBook" (although Apple probably would come up with a different name)? In all three models (11", 13", and 15"), you'd have a choice of processor, amount of RAM, and SSD capacity. The 15" either would only come with a discrete graphics adapter, or it would be an option, since presumably the integrated graphics adapter in Broadwell and subsequent processors would be powerful enough for the 15" for all but the most high-end video-processing tasks.
As for the marketing value of having a laptop with "Pro" in the name, I'm not convinced it will be an issue in two years. One, Apple has been focussing increasingly on consumer products, since that's where most of the money is. Two, the power of so-called consumer products already has surpassed what most consumers need, and plenty of pros are using them. How many pros avoid buying an MBA or an iMac just because those machines don't have "Pro" in the name? They buy based on whether the machine will do what they need it to do.
As for me, I'm a high-end amateur user (Aperture, Photoshop Elements, and Logic, in additional to the usual web-surfing, email, and word processing), and my early-2008 15" MBP has been slowing down significantly over the past year. I'm going to wait for Haswell before I upgrade. I love the screen on the 15" rMBP, but I'm leaning toward a refurbished 11" or 13" MBA and a Thunderbolt Display, since I do most of my photo processing on an external monitor (a 21" Samsung LCD I bought in 2001, which isn't as bright as it used to be, but which has served me well).