A good deed done for selfish reasons is still a good deed done. Frankly I'm just happy to see Linux gaming getting any kind of attention as it's the only reason I still dual boot Windows.

The more charitable interpretation is that Windows as it‘s currently handled is an unpredictable mess that you don’t want to be dependend on.

I think the intent is just to have an out for when Microsoft inevitably kills off the Windows personal desktop, either intentionally in favor of a subscription-based cloud service or through simply making it so crap that extremely desperate people turn to Linux instead.

Valve's behavior suggests that they've never really sought a monopolistic position. Vive was open to everyone, there's no exclusivity agreements with their marketplace, etc. But Steam requires a platform to sell games for, so making sure there's some kind of alternative is in their best interest.