The good thing about SW is you can always choose to change things in the future. Nothing is stopping Apple to implement support Vulkan now if they wanted to. It's not like they can't afford the engineering effort.

Windows supports both DirectX and Vulkan.

...and with those translation layers, you get "free" coverage of DirectX 9 up through most of 12.

So someone either rewrites DXVK to support Metal (fool's errand) or we add Yet Another Translation Layer to the mix. Something tells me the industry and users would be better off if Apple supported both Metal and Vulkan at a hardware level. Seems plausible for a company of their size.