Metal is universally supported among Apple's product lines.

Just as useful as DirectX being universally supported in Microsoft's product lines.

The point of OpenGL was to have an API universally supported everywhere

That's a bad analogy

How so?

Both are os-specific graphics apis. It seems pretty comparable to me.

Wrong

Could you explain how?

Why is it a bad analogy? I don't have experience on either of the libraries, so I may be missing something.

Everything is universally supported in a small enough universe.