While some big developers might find it worth the investment to use metal, most of us won't. When you're already invested in OpenGL and you're a cross-platform developer, Metal is not even open for consideration. Vulkan maybe, once it's more established.

What I would really benefit from is updated OpenGL support. Right now, MacOS X is the lowest common denominator, with all the other platforms having 4.5+ as standard. I can upgrade to that with ease. But I'm never going to be able to justify rewriting the stuff with Metal, because it would mean a complete parallel implementation just for MacOS, since we would need to retain OpenGL everywhere else.

Vendor-specific graphics APIs are, for the most part, a legacy of the past which we were lucky to escape from. Apple creating Metal is an anachronism, and I don't think it will have a long life unless they make it portable.

Eventually they might. Or at least if they'll be relevant at that time.