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.