Saving user's work to IndexedDB turned in to a real problem for us for a similar reason: sometimes users go in to their browser settings and click an option with wording like "clear cookies", and then it also wipes IndexedDB and all their saved work. I don't know if that applies to OPFS, but saving to the real file system (rather than just OPFS) seems a lot safer, and also integrates well with other tools, like backup software.

I've also long felt that "cookies" is a really poor term - these days it really refers to "storage on your local device accessed by websites", and the confusion literally leads to data loss.