Yeah, a file system snapshot is another way of achieving safe upgrades. There are upsides and downsides of all approaches though.

Rolling back to a btrfs snapshot might be harder than just picking the other ostree image in GRUB. And you still have the problem where old binaries might be running when only the new binaries/libraries exist on disk. But it also saves space on disk compared to an immutable distro, and it’s much easier to make changes to system files. Every approach is a compromise.