Of course the AAA game releases of late are because of SteamOS. Before SteamOS there was no "target version" of Linux for people to develop for, there were way to many variables to try and cover for a tiny share of the market, Valve have done a hell of a lot of work getting companies to make drivers and target Linux for development, even so Linux is still a tiny fraction (well under 5%) of the PC market so it's not at all surprising that we've never been given a second thought.
The SteamOS push by Valve benefits both SteamOS "console" users and us regular Linux desktop users, just because something is targeted for SteamOS doesn't mean we can't run the games on whatever distro we choose, the simple fact it is happening at all is thanks to Valve, they are the ones driving this forward by providing a standardized set of hardware and pushing companies forward for better support as well as making massive pushes to OpenGL/Vulkan. This is unheard of in the history of linux gaming; Hell, most big advances in the Linux eco system come because of huge companies pushing their agenda, we just happen to be bystanders benefiting from it.
Of course they have their own reasons for doing this but that doesn't mean the rest of the gaming world isn't looking on and taking notice when the PC digital distribution market leader by a country mile starts pushing Linux, if Valve succeed in getting a whole load of Steam machine into people's homes then I would put money on other big named companies paying attention to Linux because the market share is there to make it financially viable.
Blizzard may well have had a Linux client for years but I doubt they're holding it back just because they can it's far more likely that they have held it back because the market share of Linux is still tiny and they would have made an assessment working out that at the moment it's just not cost effective to maintain a Linux client. With SteamOS opening up Linux as a viable platform this may change in the future.
I know I'll be finally retiring my windows gaming rig in favour of a Linux rig as soon as critical mass of games I want to play are available, not SteamOS with a shitty DE slapped on but whatever version of Linux I fancy because these Improvements made by Valve for SteamOS benefit every single Linux gamer, Steam user or not, hell Valve may even cause the fabled "year of the Linux desktop" to actually happen due to this.