Valve wants a boat that is independent of microsoft. Not to go down with that Tit.A.I.nic seems like a smart move.

Exactly, and they've not been quiet about it. It's why Steam works on Mac and Linux and they work so hard on being independent of all of those.

And Arm is next.

Hardly when their business depends on running Windows games on top of Proton.

Independence of paying Windows licenses or Microsoft store taxes, sure.

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.

If Windows tanks their fountain runs dry.

What is the scenario where windows becomes so unpopular, computer games stop being made entirely instead of another OS filling that gap?

The doom that is repeated all the time on Linux forums, or even here.

The industry will adapt quickly, especially the part that's using multiplatform mainstream engines like UE/Unity.

Lots of new/recent native MacOS releases nowadays: https://store.steampowered.com/macos

The same that support Linux and yet Valve has to come up with Proton.

Developers chase the user base. If and when the users choose Linux developers will target Linux.

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.

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.

Sure, if the goal is like doing retrogaming with Windows games as if it was WinUAE.

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.

Really, where are those Linux builds?

Hmm, citation needed on that one imo. Consensus is that their hardware strategy is in service of selling more games. Hardware revenues for Steam Deck are proportionally tiny; Frame and Machine aren’t going to meaningfully change that.