Swift does run on other platforms. Though I don't know how it does on Windows.
Metal on Windows would not help much because the GPU hardware is what's different.
Metal on Windows would not help much because the GPU hardware is what's different.
A metal compatibility layer would encourage developers to start using it for games etc. Doesn't really matter if it performs a little worse - it just has to work.
Apple shipped Intel Macs with Intel/AMD GPUs until quite recently, so hardware might not matter significantly.