I was talking more about Vulkan's second-place position on Windows than about Linux (Vulkan is likely to have some advantages on Windows, like being less tied to the Windows roadmap than DirectX, or indeed OpenGL), and if Apple provided bad Vulkan support I think they'd just be letting themselves in for bad Vulkan-based ports. But overall I don't think we're really disagreeing very much here.
(EDIT: see below.)
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].
[1] http://store.steampowered.com/hwsurvey [2] https://community.amd.com/community/gaming/blog/2015/05/12/o...
If Apple supports Vulkan but with worse drivers than Metal, I'm not sure that's any better. OSX has roughly four times the penetration of Linux in the latest Steam hardware and software survey. I'm not sure it's worth it for most developers to deliberately target the second-best choice on OS X in order to pick up the handful of Linux gamers who can't/won't dual-boot Windows.