Things take TIME in the software world.
Mantle as said is turning into Vulkan. There is a REASON that we need an API like Vulkan and that is for scenarios where DX12 won't work.
- SteamOS
- other Linux (possibly PS4 later)
- mobile
- other hardware that DX12 won't support.
As said it takes TIME, but Vulkan and the game engines, tools and extended ecosystem will continue to improve.
Making a game will continue to get easier to the point that we MAY see fairly easy development of certain games to mobile, W10, SteamOS, PS4 and XB1 all at the same time (most of the core code at least).
I don't know if Microsoft can force DX on the XB1 (they do want to have XB1, W10 PC and mobile W10 devices in their own ecosystem), however it's possible Sony may allow Vulkan once things improve.
I'm not sure if there's any financial incentive for Sony to discourage Vulkan. If the overall cost is lowered (they develop the PS4 API, game integration etc) and the reliability and performance is as good or better I suspect they'll just switch over.
That would be HUGE, and Microsoft would be force to justify DX on their ecosystem with the obvious ramifications being if you don't need Windows then how many would go with Linux instead (especially with SteamOS).
*On a side note, people speak about the "failure" of SteamOS and I think that it will be a huge success. However, like Vulkan, these things take time. I think the bigger question is:
What would it take for Vulkan and/or SteamOS to be failures?