That does sound like a problem Apple could solve if they wanted to. I vaguely recall that some PC motherboards came with a duplicate BIOS, so if an update broke

That does sound like a problem Apple could solve if they wanted to. I vaguely recall that some PC motherboards came with a duplicate BIOS, so if an update broke it, it could fall back to a clean copy. AFAIK, some smartphones do this as well. Of course, if Apple manages to not break their OS with updates, that's not a problem in practice. I don't have a lot of experience with macOS, so I have no idea how problem-prone Apple's updates are.