Windows supports Vulkan just fine. People use DX because of Xbox and tooling - the existence of yet another API won't change that. And neither will WebGL be better on macOS than MolenVK.
Yes until it doesn't or vulkan implementation is considered as second class citizen. Been there seen that. The other thing is that webgpu is slightly higher-level so you do not have to go into extremes with memory handling and synchronization as in vulkan. So unless you absolutely need to squeeze everything out, webgpu gives nice balance.
