It's both, and leaning farther into fault territory.
You seem unable to imagine a world where DX and Mantle can coexist. Now if Mantle becomes so successful that most game devs won't even want to bother with DX, then either MS will have to pull up their socks, or the likes of Nvidia will have to get on board with Mantle. If Mantle does little in the gaming world, then nothing will have changed and we'll continue to be stuck in DirectX mud.
But most likely, Mantle is going to push gaming forward it is the inevitable progression of things. And more importantly (also very concerning for MS) Mantle is not specifically tied to Windows.