While Metal might be easier to use, i'm pretty sure it is still easier to have to worry about Vulkan alone than Vulkan+Metal. And Metal predating Vulkan is really only of concern to code that existed before Vulkan was made available (which wasn't that much).

OpenGL is easier to use than both, but we left it in the dust as it doesn't match the semantics of modern hardware.

Metal DOES... but only apple hardware.

Well, ya, it's a proprietary walled garden solution.

Mantle predates either, and Vulkan is based on Mantle.