I broadly agree with what you are saying here, but, I think you are overlooking something.

The Steam Frame.

It has its own internal ARM chip, is capable of running smaller games entirely self contained within itself.

And… as Proton is a translation layer (FeX) that makes Windows games run on Linux… Valve has been massively either themselves aiding development of, or funding development of a similarish kind of translation layer that converts x86 calls to ARM.

Which can also work with Proton.

So that means that a Steam Frame is technically capable of running many .APK (Android) games.

So… while things are tumultuous right now with the RAMpocalypse… Valve is laying the groundwork, in technical capacity, for mobile games to be broadly playable on its hardware, so its much less of a stretch to imagine that they could at some point open up an APK section on Steam’s storefront.