I wonder - what's wrong with supporting just the Steam runtime and that's it? Why deferring to Photon/Wine at all?
Valve should just partner with Canonical and release Ubuntu support.
Let the other distros figure out how to get it working there.
Most hardware and software vendors primarily target it.
It might not be hip, but it's what everyone knows.
When your game runs under Steam runtime, the real distribution is (almost) irrelevant - everything in your address space is supplied by the runtime, the things you get from the host system is the kernel/kernel modules and services you talk to via IPC (i.e. X11/Wayland, Pulseaudio).
It solves the problem of what version of what library is installed (if at all, maybe user removed it as "bloat") on the host system. You get known set of binaries that you can test against / coherent SDK target like with Windows or Mac.
Whether this matters is up to the developer. But it's a potential downside.
Gog is quite contend with just Ubuntu being supported; they are not that different.
Not sure whether that is the only reason why some games are not on Gog though; often Mac ports are missing too. It seems more like missing rights for the ports than technical reasons.
I don't know; probably not. But this was the response I got back from the devs of "Expeditions: Conquistador" when I asked if they could release the Linux version on GOG (when I bought it originally I still had a Windows machine).