> rewrite d3d9.dll to emulate the API in user mode on top of D3D12.

There seem to be such a project actually, just released to open source:

https://github.com/microsoft/D3D9On12

https://devblogs.microsoft.com/directx/open-sourcing-direct3...

But it seems you have to actively enable it in the development time, the OS won't just silently do this on itself and I doubt Microsoft or anybody else is going to update their software to do so.