I think this is the wrong approach for a 3D API that should be usable by more people than a very small group of highly specialised rendering engineers. Metal is an API that fills that gap very well, it is easy to use and is a modern low-overhead API.
A proper modern 3D web API should be agnostic to the underlying native API (a bit similar to how WebGL runs on top of D3D on Windows, but with much less 'conversion overhead' since most state is compiled into immutable objects).
But simplicity and ease of use should not be guiding the decisions, the emphasis should be on providing a way to run Unity, Unreal, and other engines (new and old) on the Web.
Even if Vulkan is a difficult API to work with, a low level 3d API should be available in a browser. It could be used as a building block to create frameworks and engines that are easy to use.
Even WebGL is "too low level" for most application developers, which is why three.js and other frameworks are popular.
I'm sure the spec can still be changed to be a better fit, though.
Either that, or in addition to WebVulkan you need WebDX12 and WebMetal.
http://lmgtfy.com/?q=vulkan+metal
First result: "MoltenVK is an implementation of Vulkan that runs on Apple's Metal graphics framework on iOS and macOS."
I don't think you can really put those aside - that's the whole point of web as a platform - that visiting a link won't let you take over my PC (and that's hard enough as is in practice).
Just use native apps if you want native access.
Also a reason, why outside communities like HN, most game developers don't really care about API wars.