Hi Andy,
It would make sense to launch these two together. One could be the announcement and the other would be the "one more thing"
Well, first of all, Apple can't release an rMBA because the Airs don't have enough space for a bigger battery and a fatter display.
Second, I agree that a 13" rMBP could take away sales from the 13" MBA. But it will definitely completely kill off the 13" MBP. Which is as Apple planned it: it's clear they did nothing except the absolute minimum to disguise that the 13" was a dying product that happened to bring in loads of cash. Exchange the motherboard for IVB ? Sure, now we're updating everything to IVB. we can do that at the same time. Update the display or anything else ? No way. Easy money while the engineering resources are focused on the real follow-up: the 13" rMBP.
I expect the 13" MBP without retina to continue existing until the sales numbers don't make it worthwhile anymore, which could be as early as Q1 2013. The product line will then be a 13" MBA starting at 1280 and the 13" rMBP starting from possibly 1600 (8GB/256GB). The 15" MBP will continue in retina and non-retina versions as it does at the moment.
Peter.