shmerl: I trust actual developers who work on this more. And they contradict what you said directly. Unless it's a game using Stadia specific functionality (which CP2077 is not), it's simply a Linux port.
I think what he meant is that a Linux port will most likely work on Stadia, but a Statia port (i.e. optimized and only tested on) will probably be a broken Linux port. It even happens on Windows when some developers only optimize and test their game for NVidia (or AMD) card and it end up being totally broken on anything else.
shmerl: Check what actual developers say about it. Controlled or not, they simply make a Linux port. Amount of testing is limited for sure, but still. Same port can be used for running on regular Linux, granted they test more.
The "controlled" part is what makes all the differences, it doesn't matter if the code is 90% the same, having a controlled environment means you can test, debug, optimize for a specific hardware, for a specific OS and totally ignore the rest. And it gives you very little support.
That can make the difference between a viable platform and a non-viable one.
shmerl: It doesn't matter what they are interested in it for. Install base is what matters and Valve estimates demand in millions for it. I bet it will be way higher than Stadia.
No, the only thing that matter is Linux gaming market share, it doesn't matter if Valve sells hundreds billions of Steam decks, if only a fraction of them are used for Linux gaming it's not going to do anything.
To go back to proton the ironic thing in all this is that Proton was originally created by Valve to "protect" themselves against Microsoft in case they decided to lock down Windows.... except Microsoft doesn't give a damn, they only care about Azure and cloud.
But on the other side, thanks to Proton it makes it a lot easier and cheaper for Google or other game streaming companies to offer games as they can do it on cheaper Linux infrastructure.
So by trying to counter Microsoft, Valve ended up helping streaming which is probably a much bigger thread against them than Microsoft ever will.
I wouldn't be surprised if in a, hopefully, distant future all new games will be running exclusively on Linux... except they will be only available via streaming.