Give it a few more OS releases and Android will be the only one supporting GL.
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.