Right now even Valve realizes that Steam will literally run out of steam. This is why they have been trying to become more like Nintendo and selling their own hardware (with varying success) .
Independence of paying Windows licenses or Microsoft store taxes, sure.
There's tons and tons of older software that people still want to run that might never be ported to Linux. And that's fine, because there's no problem with building compatibility layers to make it work. Microsoft can't do anything about that.
Lots of new/recent native MacOS releases nowadays: https://store.steampowered.com/macos
Proton as a project let's valve hedge on the heir apparent OS without upfront developer cost. If the Linux player base grows, developers will follow and valve is poised to remain dominant.