There's nearly always advantages to being the owner of the complete stack, even if developed in the open and allowing other backends. Making sure that every piece matches your vision without little weird API impedance mismatches for a library being used in many places is a massive win. Nobody expects SwiftUI on Apple to be crippled by a decision, even if it would make SwiftUI on Windows and Linux better.
That's one of the reasons AMD pushed Mantle, then pushed Vulkan and DX12 and dropped Mantle. The shape of the API better fits their model for driver development than OpenGL or DX prior to 11, and they're happy for their competitors to help build an ecosystem that works better on their products than the alternatives at the time.
IMO, this is the best model for library/API development if you can pull it off.
That's one of the reasons AMD pushed Mantle, then pushed Vulkan and DX12 and dropped Mantle. The shape of the API better fits their model for driver development than OpenGL or DX prior to 11, and they're happy for their competitors to help build an ecosystem that works better on their products than the alternatives at the time.
IMO, this is the best model for library/API development if you can pull it off.