The point is that Proton puts them in a win win position. If Windows stays popular, they're fine. If Windows tanks, they're fine.
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.