If you want to argue Apple is handling this poorly from a community perspective, you'll find no argument with me.
The reason Apple is doing this in this poor way is that they seem to want to push Metal as the answer. They want incentives for developers to move to Metal by making their lives more difficult on OpenGL, because even with libraries those libraries must be maintained and included. They don't want to bear responsibility for this legacy. I'm explaining, not defending.
This is the direct opposite to the Linux ecosystem, where Gallium3D not only facilitates both OpenGL and Vulkan, but has had attempted Direct3D implementations in the past. Yet another case where Linux has the "everything and the kitchen sink approach", and Apple just flat out dictates a single choice to everybody. That attitude sometimes helps Apple, but sometimes it doesn't.
Mesa Gallium 3D is not an API intended for use by applications. It is a mechanism that allows to share functionality among drivers. Yes, it is possible to create state trackers on top of it, but that will be part of Mesa. It has no public API.
Even among Mesa drivers, usage of Gallium 3D is not mandatory. If the driver team thinks it will make their job easier, they can use it. If they think it won't, they won't. Intel 965 driver (the current driver for Intel GPUs) doesn't use Gallium, for example. The new one ("Iris"), currently in the works, will. For a long time, the only driver that used Gallium 3D was only the AMD one.
The reason Apple is doing this in this poor way is that they seem to want to push Metal as the answer. They want incentives for developers to move to Metal by making their lives more difficult on OpenGL, because even with libraries those libraries must be maintained and included. They don't want to bear responsibility for this legacy. I'm explaining, not defending.
This is the direct opposite to the Linux ecosystem, where Gallium3D not only facilitates both OpenGL and Vulkan, but has had attempted Direct3D implementations in the past. Yet another case where Linux has the "everything and the kitchen sink approach", and Apple just flat out dictates a single choice to everybody. That attitude sometimes helps Apple, but sometimes it doesn't.