It's not only goodwill, it's a survival tactic for valve. The worst case scenario for them, is see microsoft expand their monopoly to have all apps and software available only through their Microsoft app store, competing directly with valve, with the unfair advantage of microsoft controlling both windows and the app store. They could (and probably have) tried to get to where apple is with its app store on macos/ios. Though of course this would be an anticompetitive move, but the intentions could still exist making valves life difficult.

The moment they can untie gaming from windows, they have a path forward to keeping themselves not only alive but relevant and probably safe.