MoltenVK VS MoltenGL

https://github.com/KhronosGroup/MoltenVK

Translating between rendering APIs is not really the problem. The GPU design is more different than the API is.

Khronos maintains MoltenVk though, which is "official" as it gets: https://github.com/KhronosGroup/MoltenVK

...technically, Vulkan on Windows is also only supported via 3rd-parties (the GPU vendors), Microsoft doesn't support Vulkan either ;)

MoltenVK implements large parts of Vulkan on top of Metal for Apple systems. It isn't full Vulkan but it makes porting Vulkan games to OS X easier.

That's cool. Maybe I haven't thought enough about this. Let me check it out. FWIW it's this PR that you are referring to I think: https://github.com/KhronosGroup/MoltenVK/pull/1815/

For Metal specifically, they could adopt and contribute to Vulkan and get access to a lot more software. Right now you need to use a compatibility layer, and surely Apple could just support both APIs natively with much lower overhead. But they don't, because it nudges developers to stick to the Apple ecosystem instead of being able to support multiple platforms.

At the moment the most promising thing is MoltenVK (DX12 -> DXVK -> Vulkan -> MoltenVK -> Metal), but the development is not that quick mainly because there aren't tons of developer that works at the same time on the project. It's not actually a Metal related problem at the moment (they have a road map of things that they can be achieved with Metal 3 like Mesh shader and Geometry shader).

What you asked is already existed for at least 8 years. Yeah EIGHT years. It's called "MoltenVK". So far it's the only implementation of Vulkan for macOS. Basically it's a wrapper that runs on top of Metal API.

Despite championing OpenGL back when DirectX was their main competitor, now their OpenGL support is deprecated (i.e. it works but has not had new features in close to a decade) and they're expecting everybody to switch to Metal. There exist some compatibility layers like MoltenVK and MoltenGL that were developed or acquired by the OpenGL/Vulkan group, but they do have some limitations. Apple is not helping with their development at all, either, and will not provide developer support for using them.