>If GOG went down tomorrow, those games would be gone just the same as the Steam ones, since I gave up trying to back them up a decade ago.

That's a you problem. I keep mine backed up. GOG even makes it easy to just go grab the latest patch if you downloaded the base game years ago.

>None of my GOG games run on Linux, while most of my Steam games run on Linux. Also annoying: None of the GOG games run on a genuine Win9x either, as what you are getting is an updated version for modern Windows, not the original game.

Solvable with Wine/Proton (or a Proton fork). If you want to be lazy Heroic launcher makes it easy and even logs into GoG if you don't have them backed up locally. Heroic launcher lets you pick what you want to use to run the game whether it be Proton/Wine/Etc, this even works with Game-Porting-Toolkit on MacOS.