I watched all the relevant WWDC Metal talks and they look pretty serious to use it as their only graphics API, with OpenGL left in its current state for backwards compatibility.
Unless Apple proves me wrong by adding Vulkan support, I would say they were on the meetings more to learn how to improve Metal than anything else.
Would it be feasible to implement Vulkan atop Metal as a library without horrible performance? (Keeping in mind that the baseline of Apple's OpenGL is already fairly slow and behind the times. And Apple's customers already have fairly low expectations wrt gaming/etc.)
Unless Apple proves me wrong by adding Vulkan support, I would say they were on the meetings more to learn how to improve Metal than anything else.