Proton is a downstream fork of Wine, and upstream Wine already directly supports playing Windows games on Mac using D3DMetal.
You don't need Proton's Wine fork when you can just use Wine.
You don't need Proton's Wine fork when you can just use Wine.
There's also DXMT which is open-source, but doesn't support DX12.
Wanting Proton on Mac isn't about that specific fork of Wine, it's shorthand for wanting the user experience that Valve gives you on Linux.
In comparison the proton experience is seamless.