They could do it, though. They could.
All major middleware engines already support Metal as well, and relevant companies like Adobe are using it as well.
Even if they don't officially drop GL, they could just leave it on life support, similarly to Carbon APIs.
And neither Direct3D nor Metal have any real advantages over Vulkan. They exist for locking in developers.
Sony did bother to offer OpenGL ES 1.0 with Cg for shaders. Hardly anyone used it.
Nintendo introduced Vulkan on Switch, while keeping their own API. Lets see how it will go.
The advantages over Vulkan from both APIs are not being C based in 2017, great graphical debuggers, a full stack experience instead of tracking down libraries for math, fonts, textures, ....
Game development culture is not about being all friends and sharing code, rather making the best money of IP and selling services, in particular porting code among gaming devices.
And then as a regular joe you would actually put in the effort* to get things working well. Then you would watch these bug-plagued big name 3-star ports appear out of nowhere and usurp you on the top free lists and search results due to their favorable placements. And then you would move on to a fairer market, and when microsoft stopped paying them, the big app developers did too (not that they ever updated or fixed the bugs on windows phone releases.)
* Difficult as windows phone was the only mobile platform not supporting openGL in hardware and the top sold windows phones were incredibly low spec.