It may be the case that people value the mobility so much that Apple isn't dropping the price on the MacBook Air. (Or, alternatively, people are no longer buying the heavier MacBook Pro, so they've dropped it's price.)
(I speak as a former user of a 2010 4gb/256 SSD MBair myself. Couldn't go back to it, these days.)
I travel a lot, and even the 15" rMBP is not a "deal breaker".
It's funny how pathetic it would look to our fathers that we find 2.5 kg to much to carry around in a bag...
Just to put this into perspective, a medium sized t-shirt usually weighs 6-10 ounces (160-285 grams) approximately.
I travel for business quite a bit (20-50,000 airmiles/year) and I traded in a 13" MBA for a 15" rMBP upon its release.
For me, it was an easy deal to exchange the laptop weight differential through unnecessary clothes or travel accessories for this gorgeous display and added (dev) computing power.
I have deployed several of each: 2011 and all subsequent MacBook Air notebooks, and every model of "MacBook Pro with Retina Display".
The Airs are inferior in most ways, but the Retina machines crash. Kernel panic, or also GUI death (where you can still SSH in but there's no more GUI until you /sbin/shutdown -r now).
Reliability is the magic sauce that makes computers awesome or not awesome... too bad we all don't have access to the telemetry that Apple and MS do. We just have the anecdata from the interweb tubes.
But IME the current generation Airs are pretty awesomely reliable, as far as notebooks go.
(Having said all that, I still go with a maxed out 15" Retina at scaled effectively-1920x1200, even though it does both ghost and crash every couple weeks. I just can't use those legacy LCDs anymore.)
The MBA has a 2-core i7-3667U-2.0GHz while the rMBP has a 4-core i7-3615QM-2.3GHz. There may be a "540%" difference between the highest performance rMBP (4x2.7) and the highest performance MBA (2x2.0) when naïvely considering just clock speed and cores.
However, while you have a point about the weight difference having shrunk to only 300 grams (or 1/2 lb) with the retina version of the MBP, I think the performance difference won't be noticeable to people who aren't performing 'exotic' and computationally intensive tasks.
The new integrated GPU's power and greater energy efficiency may make a retina display likely on the Air, so I see the segmentation becoming pretty obvious and increasingly based on specific users' profiles rather than budget alone. For example: are you more interested in portability and probably battery life; or, are you doing some compiling or scientific computing away from your desk?
13" Retina Probook with 2.5GHz i7 processor, 8Gb RAM, 512Gb SSD, costs ~ £1650.
13" Airbook with 2.0GHz i7 processor, 8Gb RAM, 512Gb SSD, costs ~ £1600.
So for an extra £50, you can go from the Airbook to the 13" Retina Probook. You gain a vastly better display and 20% faster CPU, in return for a 300 gram weight penalty.
My reading of this is that the 13" rMBP has massively eroded the value proposition of the high-end 13" Airbook. So either the 13" Airbook price is going to have to come down significantly, or the spec is going to have to rise, or both. (Something like a 2.5GHz Haswell CPU and a £50-100 price cut would begin to make it appealing again.)