With Google and Apple already controlling the mobile app stores, various console manufacturers controlling their own ecosystems, and Microsoft attempting to abandon their "open" Windows (so they can run their own app store) - the industry is shifting into a position of hardware/OS manufacturers/monopolies taking massive cuts (30%) from the software market. Valve, without any hardware of their own, but a very large PC app store with a significant user base, really has nowhere else to turn. Gabe himself has expressed quite a bit of dismay over Microsoft's recent decisions.
I expect them to use everything in their power to make Linux become a mainstream thing. They're going to port all of their own games to Linux, and I expect at some point there will be Linux-only discounts for their games.
I really hope Microsoft underestimated their power, and that Valve is capable of doing this. It'll be better for all of us. The software industry is heading in very bad directions, and this is at least a glimmer of hope.