I think Mantle will end up like AMD's Stream, it will die by the end of this year.
But that's okay, it's not a bad thing. It's better for everyone that this happens because it is going to be displaced by DirectX 12 and OpenGL's low level API's. In that regard Mantle did its job, it brought low level API performance to the top of everyone's minds and has pushed the industry in a new direction.
Everybody wins. Consumers get a graphics agnostic solution so you don't need an AMD graphics card, game developers get better performing product to sell us and graphics makers have a new feature to trumpet.