I believe the purpose of Zink is that if someone is writing a new driver for a new GPU, they only need to write a Vulkan driver. After all, OpenGL is a legacy API. Still has lots of uses. But, I think it's better if the effort to write a new driver is spent on bringing up just a Vulkan implementation instead of needing to write two drivers of once.
It's kind of like how Mesa's existing OpenGL drivers all switched to Gallium3D, which splits the Graphics driver into an agnostic front-end and a device-specific backend. Allows for a lot of re-use and optimization. I heard they're wanting to do something the same with their Vulkan drivers for the future as well.
Someone could write an OpenGL layer directly on Metal. There have been a few incomplete starts at that.
Meanwhile, writing an OpenGL-on-Vulkan layer has been a plan for deprecating/simplifying driver support for OpenGL for quite a while now. Might as well focus efforts on that.
