A lot of people on the thread are focusing on how hard/not hard it would be to make a wholesale transition of Cocoa from one language to another ... I'm coming from the world of Windows, which might offer an instructive history. In short, Microsoft would think nothing of making a change like this.
The Win16/Win32 API was a thing of beauty and was the whole universe for several years. It was written in C.
Then MFC came. It was the new hotness in the late 90s and it was based on C++.
Early 2000s. Goodbye to C/C++. If you want to build a native Windows app, you would probably think first of C#, which used a completely new, non-C Windows API called Windows Forms.
Today ... want to write a Windows app? Visual Studio's new primary language for you is Javascript, talking to a complete javascript-native API for windows.
And let's not forget Visual Basic, the most popular language in the world, which has its own Visual-Basic esque toolset and Windows API.
Microsoft certainly has a much bigger vision for its development tools, and a much longer history. Its very first product, after all, was a programming language (MS Basic) - and developers have in a sense been a much larger target market for them than they have for Apple in their shared 30+ year history.
But now Apple, with its skyrocketing income, surely must be aspiring to the herculean scale of Microsoft's year-in/year-out dev tool undertakings.
My bet is Apple will come out with a Swift-native version of Cocoa within 18 months or so. ObjectiveC Cocoa will be supported for a LONG time (I believe all the above MS dev technologies are still usable in some form or another ...)