AFAIK, the PS4 uses a proprietary and custom Low Level API and absolutely not Open GL. source. I wouldn’t mind to be told wrong if you have the sources for it though !
No argument there. Linux represents 0.83% of the PC market when it comes to OS, Mac around 3.3% (but as someone said, there ain’t really the hardware). As for the source, they are here.
PC Gaming has been, on games like TW3, about 30-35% (source). 5% of 30% is 1.5% potential revenue… So there are no arguments it’s not viable economically.
But that’s irrelevant, there ain’t any argument either on the fact Warhorse should never have mentionned Linux / Mac Os X in their kickstart, unless they knew whatever they were planning would be compatible with it straight from the start. I can’t speak for them and tell you what changed (if anything) for them to scrap this, but unfortunately, in 2016 releasing games for Linux / Mac OS X is still very much an act of militant-ism/philosophy rather than a purely benefit guided action. Even for AAA companies, 1.5% market share is still very limited.
I’d be very happy to see more Vulkan games, but I believe the only guys who can actually make this happen is Valve, through encouragement via Steam. Higher revenue for developers if their game work on Linux natively (via OGL or Vulkan) would probably help plenty !. Oh, and Microsoft can actually make it happen too, if they continue to do the wrong things with the UWP.