If you think this is definitely going to end mantle, I think you don't appreciate the complexity of the situation. Firstly, opengl isn't relevant, so let's get that out of the way. With DX12, MS will historically force you to buy a whole new OS for the privilege of using a new API. You'll also have to buy a new gpu most likely, whereas mantle can work with existing hardware. DX12 doesn't seem like it would work on maxwell, so you would have to wait for a new nvidia architecture to be released before you could use it, but kaveri might retroactively support it due to similarities between it and the xbox one.
So you effectively end up with the same sort of dynamic, the low level api that only works on amd cards vs dx11 nvidia cards.