What about cannabilism? If the MBP 13 inch is lacking an ODD, then it will cannabolize and be cannabolized by the MB-Air. I don't see apple setting up a MBP that does not have the biggest feature missing from the MB-air.
Cannibalism? Yeah, what about it?
The Pro will still be more upgradeable than the Air, crunch numbers better than the Air - and probably have discrete graphics. The Air, however, will still be lighter, thinner and snappier than the Pro.