But I would like to mention that people seem to overestimate the work required to "port" a framework from one language to another. Take 280North as an example. A tiny company (funded by ycombinator) founded by 2 ex Apple employees. They have developed a JavaScript variant of Objective-C (called Objective-J) and reimplemented the most important parts of AppKit. It took them a couple of months but in the end it all worked out, many people used (and are still using it) Cappuccino (name of the framework) and 280North was bought by Motorola for something like $20.000.000. So two guys => 1 language, a couple of frameworks in a matter of months.
Also I am sure you are aware that every year Apple introduces a ton of new frameworks and deprecates some frameworks. It would only take a couple of releases to deprecate 80% of the frameworks used by most apps and introduce replacements for them.
Swift without a new set of frameworks makes little sense IMHO. So either Swift will dies - just like GC did or it won't and then we will see new frameworks written in Swift.
Talking about GC… When Apple introduced GC they told us that it was the latest and greatest shit. It was the future. When introducing GC they told us that "hey - we have ported Xcode to GC - look it is that easy.". So they had more apps using GC from day 1 than using Swift from day 1 (well the WWDC app written in Swift does not count…). Apple had to ship GC-capable versions of their frameworks from day 1. They were even more committed to GC than they are now to Swift so it would not surprise me at all to see Swift let go in a couple of years and buried deep.
At the moment everything seems possible.