Agreed on not being sure that it's Dropbox's fault. But even if the user killed the file, you've got the same problem.

A person running Time Machine (or similar incremental backup system) is a lot safer from this sort of problem than a free tier Dropbox user. Free Dropbox is better than nothing, but people can't keep assuming their files are safe because "they're in the cloud" and get synced to a few places.