Metal is an unnecessary complication.
The fact the you want Apple to support other platforms doesn't mean that they're wrong for not doing so. They same arguments to require Apple to support your preferred API also applies to MS, NVIDIA, AMD, etc and the same arguments to not do that apply them all as well.
So GL and CL had to make a compelling argument for why apple should continue to invest in the technology, and even now they still aren’t able to show a single compelling argument for them being better than any alternative.
Personally I would have rather a single open API, but the open API did not appear to be super interested in competing with the other platforms. The end result is the open platforms turn into a pile of expense, licensing, and engineering for behind the curve technology and API.
But native OpenGL requires apple writing OpenGL implementations for their hardware. It requires apple spending engineering time implementing misfeatures from OpenGL and opencl in order to support APIs that are only every considered the backup API.
And again, anyone could have made a Vulkan emulation layer on metal.