Sure there'll be the occasional hobbyists but they're not really what drives open source development. In my opinion.
Too bad your opinion doesn't correlate with reality.
Yes, there are open-source projects that are backed by companies, but there are also many successful open-source projects which are primarily community-driven. Both are significant sources of development effort, and neither should be underestimated.
So what's the model really?
Valve will take a cut of games they sell via SteamOS.
Paid developers will do a significant part of the work on SteamOS. They will be paid because they are games developers who sell games on SteamOS.
I never suggested that Valve would have to do no work at all, only that they would also benefit from the work of volunteers.
Game players will buy games on SteamOS.
A very small fraction of those game players will have the wit and know-how to make changes to source code. This is a volunteer coder.
I would imagine that most players on SteamOS won't want to fix bugs, as it's a commercial product targeted at gamers. This is irrelevant as long as SteamOS is based on Debian, and people who write code on Debian run Steam.