If Microsoft or Nvidia wanted to use open APIs they would just support technologies that already exist, e.g there is GPU virtualization with Virgil 3D. There also VirtIO which is standard for all kind of virtualized interfaces, but again Microsoft only interested in using their proprietary tech.

Microsoft could also just use Vulkan instead of bringing proprietary Direct3D user pace to Linux. Or might be they could actually open source parts of their stack, but then it's will be against their vendor lock-in.