The only hope for Open source software is that governments around the world discover that they can save a load of money when they use OS software. The netherlands have just decided to go OS in the future and others have done so before. If those users realise that the systems work as good or better than MS's than there's hope that they'll install Open source software on their private machines, too.
If that gets established the game developers sooner or later have to deal with those customers that use a different platform.