Okay, but stealing a backup gets you 21 days of those documents, the same amount you can get from the live system. There is no need to worry about backups in particular.

Correct me if I'm wrong, but if you had a backup from 15 days ago, wouldn't that backup contain documents 21 days prior to it? So in effect: up to 36

The backup will have documents from 36 days ago through 15 days ago: 21 days.

If you're worried about someone stealing your entire backup system then you have bigger issues.