That would either mean that Apple needs to maintain two separate types of display drivers or create a new abstract driver interface which sits between Metal+Vul

That would either mean that Apple needs to maintain two separate types of display drivers or create a new abstract driver interface which sits between Metal+Vulkan and the underlying hardware. That's just adding busywork with only downsides, both technical and organisational. Vulkan just isn't that important outside of Linux and Android, especially when most games are built on top of a crossplatform engine anyway.