I think this is a key point. While the Steam cash cow is generating enormous profits, they have a lot of freedom to experiment. They have an almost complete monopoly of the digital game download world. It's to some extent reminiscent of the freedom that Google had early on when they were absolutely dominating search, and they weren't trying too hard to attack other fields.

It remains to be seen if that's the only reason why their current structure is working, but that's at the very least a valid hypothesis.