The "language" may be "open sourced"; but will the important Cocoa implementations be open sourced?
It shouldn't be to too tough to re-implement Swift; unless Apples starting suing people for doing it.
The exact meaning of Apples announcement needs some clarification.
Cocoa won't be made Free, no, but perhaps Swift could be used with GnuStep's implementation of the libraries.
It's better to not re-invent the wheel.
I think Foundation might be necessary though, even if you just use Swift structs (Array, Dictionary, etc)
I'd love a portable GUI API that satisfies some harsh conditions: 1) Portable between mac,linux,windows,android,ios. [ must re-compile to target ] 2) Compilable into a single static binary. 3) New developments don't break backwards compatibility.