Yeah I am not sure what the extra commitment is, especially since DX12 was a reaction to Mantle/Vulkan not anything to do with Valve.
Also it seems that Valves linux support is very lackluster, they last posted on their blog in November 2012 (http://blogs.valvesoftware.com/linux/).
They setup a steam group for it and have left it at that, while SteamOS is getting updates users can't even use a GTX 1060 or 1050 series card as the Nvidia drivers are outdated.
Even the users on their forums are basically saying it feels abandoned http://steamcommunity.com/groups/steamuniverse/discussions/1/154645497345312174/
So disappointing really, even their march against Windows Store doesn't even compare to Tim Sweeney, who is more up in arms for UWP and yet Windows store in Windows 10 is so much bigger than Windows 8/8.1 ever tried.
Valve seems really big on the initial show and stating "It will be open", then they say it's Open Source and leave it for the others to deal with. Even their VR went that way, and it could be excuse if they just want to push the tech so they could make games but they aren't exactly releasing many games on a regular basis.
HDMI 2.1 having variable refresh is nice, another alternative to G-sync and Freesync (though I assume Freesync will just adapt it to work with HDMI). OLED HDR with variable refresh, things are looking good in display land over the next couple years