"There is no doubt in my mind that support for Mantle/DirectX12/xxxx will be rapid because the benefits are both obvious and easy to explain, even to non-technical people."

Except that this won't only depend on the programmers, but on the management too, and seeing what those guys do at EA, UbiSoft, Activison, etc, I am not sure it will be properly utilized for the first wave of Dx12 games.