My guess is that R&D as well as third-party Steam keys eat into their margins.

It could be more sustainable with this higher fee as well. Valve supports old games for a long time whereas console manufacturers pull the plug 10 years later. You could argue that Microsoft takes only 12%, but Microsoft has the luxury of being able to exit the PC games market at any time, or they can take a loss on it indefinitely. Valve needs to survive off its PC store because it's the only thing they really have