Because of Oracle v Google, supporting applications running in the Win32 userspace isn't necessarily leaving yourself open to threats of Microsoft meddling.

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.

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.

I believe they have proved that very few games are actually Windows games. The few remaining are mostly those which require Windows kernel drivers to run or connect to online services.