An immutable OS can be set up to revert to the previous version if a change causes a boot failure. Or even a COW filesystem with snapshots when changes are applied. Hell, Microsoft's own "System Restore" capability could do this, if MS provided default-on support for creating system restore points automatically when system files are changed & restoring after boot failures.
What's funny to me is that in college we had our computer lab set up such that every computer could be quickly reverted to a good working state just by rebooting. Every boot was from a static known good image, and any changes made while the computer was on were just stored as an overlay on a separate disk. People installed all manner of software that crashed the machines, but they always came back up. To make any lasting changes to the machine you had to have a physical key. So with the right kind of paranoia you can build systems that are resilient to any harmful changes.
