Did you mean Vulkan where you said "Mantle's second-place position on Windows?" As far as I can tell, Mantle is pretty much DOA -- Nvidia and Intel aren't going to support it on any platform, and Nvidia has 52% of the PC market according to the latest Steam user survey[1], and Nvida/Intel combine for roughly 80%. Even AMD is deprecating Mantle in favor of DX 12/"GLnext"[2].
Yes, of course I meant Vulkan, sorry. IIRC though Vulkan is effectively Khronos-standardised Mantle, so it's not all that fair to say that Mantle has died exactly.
[1] http://store.steampowered.com/hwsurvey [2] https://community.amd.com/community/gaming/blog/2015/05/12/o...