“but they could (as many companies do) keep the improvements all for themselves” I don’t know, by the license of what they use they are obliged to share the source code, if only with their clients. I’m not a legal expert, but from what I understand they couldn’t create a closed version of SteamOS. In any case, there is a mutual interest in the project, valve contributes a lot to wine, but I don’t think they would be able to maintain a project of this complexity on their own, let alone get to where they are today.