I leave my Steam update settings to only update on Play. This allows me to pretty easily review what is being patched in to any particular game and choose when to apply it.

It’s also trivial on Linux to keep the Proton prefixes and game installs backed up automatically. This has the added benefit of making all game installs portable.

Unfortunately GOG still has no official Linux client, though there are workarounds. The “apocalyptic scenario” would ultimately kill either company, so you’re right to think it is moot.