Why can’t you just roll back from a bad macOS update?

After the live CD years it became much easier to abandon the headaches and work with large signed images, but most OS vendors have had at least one pressure to continue with the hellscape of patch permutations and one way configuration adaptions.

This reads like Microsoft’s ‘the old new thing’ where they make up excuses for choices made by Microsoft and here for Apple.

I presume the real reason is that Apple would say instead of preparing for the update to fail, it’s better to make sure it doesn’t fail in the first place.

IIRC that's how Windows handles update rollbacks, by doing this. Though disk space tends to be much cheaper on Windows devices, on Apple devices you pay a premium for extra internal storage.

Anyway seeing as Apple froze GPL software at the last GPL2 version, surely you’re using Homebrew or MacPorts to get a newer version of emacs anyway?

As for C compilers, I’m not sure if they’ve ever been in the base install, but AFAIK it’s “xcode-select --install” you want if you don’t want to install all of Xcode.

[0]: https://eclecticlight.co/2020/01/23/catalina-boot-volumes/

This generally shouldn't be happening in minor updates, and even if it does, there are ways of designing apps to handle this. I've done it before. (And in fact, software developers themselves often roll back versions of their software while developing and testing, which you should know.) As the author indicates, rolling back is possible, albeit overly difficult, so the possibility must be accounted for in any case.

Major OS updates are different story. The key there is to avoid being an early adopter. ;-)

Up until recently they did support rollbacks from a bad update, so I'm not sure the above holds as a reason. They used to, and if it's harder now then that is part of a deliberate design decision.

(Seems like it got better in Big Sur, and haven't had a single one in Monterey)

For Debian you might need a file system with snapshot support, e.g. apt-btrfs-snapshot is a package that handles it, though I never used that myself.