The question is moot. Valve does not care for wide adoption. It's just another option.
SteamOS is really only good for Steam machines that are dedicated gaming machines. Having SteamOS as main OS is just not viable for the vast majority of PC users.
But you will have the option of dual booting. Which works, but some people find it annoying.
So just boot to SteamOS to play Half Life 3 whenever that comes out? They really want other developers making games for it - do you think they can, or is it already too late?
So why even bother? They could put that time, effort and talent towards making new games instead. Should they just be an Gaming company and not try to be an OS/Hardware company?