You can still build compatibility layer on top of immutable FS. Like being mutable from program(s) perspective while being able to control how these changes are capsulated from rest of the system. You could, for example, see what your

> any OS, application, library, filesystem, device driver, database, nearly everything related to computing at all

-is trying to actually do to your files and operation like reverting changes is trivial. Sure it's not performance optimal and effects what kind of software is ideal to work within such a system. It's just a different approach with different trade-offs.

Anyway if I one day want to build new OS it's for trying different interesting approaches and not yet another 'successful any-OS'.