Vulkan isn't a viable alternative for Apple. Apple wants to give developer an easy to use API to encourage them to use it as much as possible. Vulkan does not offer that.
Nah they want lock in, in the Epic Suit Epic used almost none of their libs but had to use Metal to function, and they were about to extract leverage by pulling a dev license for desktop. All POSIX and Vulkan/OpenGL and they could have stayed safe.
