Apple should just open-source the entire language (runtime, implementation, spec etc.) and let the community make it stronger. Seemed to work with Google and Go!
EDIT: C# could call into C++ from the start, and I believe the reverse was also true. So platform-wise, MS matches the Swift<->ObjC/C bridge as well. I forgot about that detail.
The problem with C# and .NET is (was) that they are patent-encumbered and that's the real game stopper. It took quite a long time and a lot of debate on .NET suitability for a Free Software movement to settle that matters somehow (recent releases under Apache license seem to fix many issues, though, although that still needs a statement from FSF's lawyers).
I'm unaware about Swift relationship to software patents. Does anyone know?
To use Go as an example, the entire Windows port was done outside of the core team, and is now a supported platform. The same goes for ARM, PPC64 etc. Also, huge performance improvements in core features such as channels were implemented by non Google employees.
If Apple open Swift, it can find hold in all sorts of niches they didn't expect it to, and can benefit from an entire community of developers who can fix the language's problems rather than be frustrated by them.