Someone will write the necessary scripts (e.g. fossil-rebase) to go into the DB and rewrite history.
No, they won't. The fossil file format is very strongly protected against any changing of history. Changing history changes multiple hashes (at multiple levels) and breaks it. Immutable history is literally part of the metadata specification, and the db is "just a data store," independent of that specification.
Shunning is a special case which removes a given artifact from the db, but is considered a "nuclear option" of last resort. i've never personally used it.
Not db rewrite, but the same end result.
edit: bad english
If you accidentally check-in proprietary or sensitive content that you didn't mean to publish, you can shun that content. Shunning leaves a hole in your history. There is no way to replace that hole with different content.