I think it all depends on what their mission is, what bits of the larger ecosystem does valve care about or not care about, what do they want to defend or destroy. I'm tempted to say "defending the store is existential for them" but the scale of it likely gives them an extremely big comfort zone. Whatever OS the users have isn't going to affect whether they can do commerce through them unless MS does something unhinged like getting rid of win32, or as the fear around win8 that they'd centralize all windows software through their channels.
If their mission is SteamOS, then I'm tempted to say they need to expand their area of influence beyond what they're doing with linux now which seems to be strictly game related, which likely involves whittling down the major or minor reasons people don't use desktop linux (and all the projects that comprise it) in general, not just their distro. So long as they're supporting (in one way or another) the majority of windows games in a foreign environment and the richness that mods can bring they could do with improving the experience of that relative to windows and not just the happy path of what happens if the base game launches. And that's before the enigma of anti-cheat. Then find some way to reap the rewards, if any, for doing that greatly expanded workload or investment.