You’re ignoring D3DMetal which is what is most commonly going to be used and equivalent to what Proton is doing to convert D3D to VK.
Most games are D3D. A very small minority are Vulkan from the get go.
On many games I have tried DX => DXVK => MoltenVk => Metal is significantly faster than DX => D3DMetal. For example XCOM2 is about twice the frames per second (yeah it has an official Mac version but it is even slower).
On many games I have tried DX => DXVK => MoltenVk => Metal is significantly faster than DX => D3DMetal. For example XCOM2 is about twice the frames per second (yeah it has an official Mac version but it is even slower).