So, by your logic, as Direct3D 8 was released before Mantle was announced then Mantle was spurred on by Direct3D 8? O.o
Direct3D 12 was in development long before Mantle was usable, probably even before it was announced, it's development cycle would have completely dwarfed Mantles as it needs to work with N times as many GPU's, it's probably been in development since before Windows 8 launched!
the deveolpement cycle for DX12 wasn't that long , since its based upon DX11.2 - the next feature level is called DX11.3 ; as someone said previously (I think) MS hasn't really cared about gaming on PC since xbox came out