Mantle will be part of a competitor even if Mantle itself loses. Because AMD is giving Mantle tech to Khronos for shaping OpenGL Next.How many developers are working on DirectX 12 right now? Do we have engines running on it validating MS's claims that the API is just as good as Mantle is in terms of multi core performance and lower overhead?
It would not be the first time a Technology war is won by marketing, F.U.D., PR, and sheer resistance to change. See Intel and their fight against the MS-led ACE consortium seeking to replace the Intel in Wintel, see all the cartel like crap against Rambus in the PC space (sure they were no saint, but their technology did work well and they were essentially forced out by the other DRAM makers).
Sure, DirectX 12 will be on all GPU's and developers will support it, but it looks like we are basically admitting defeat to any chance there will ever be a competitor strong enough to challenge or supplant the current market leader. If this does not make the leader complacent, I do not know what will .
I'm very disappointed in the Steambox by Valve, because I thought they were going to push in the direction of Linux and get others aboard so OpenGL had a big non-mobile use case again.