Three years ago, after doing YC Startup School, I built https://www.borgbase.com to offer the simple, but secure backup service I wanted myself. Today it’s a viable business and my customers are all great and value backups as essential part of their own business. Wouldn’t want to be in any other “more glamorous” corner of the industry. Also kudos to anyone - partner or competitor - working in this “unglamorous” space. You’re all doing great work.
Additional offline backups aren't really feasible past a certain data volume and daily change/velocity. I'd still encourage everyone to have them for their own essential data in addition to a cloud backup. E.g. by burning it to BluRay or tape (3-2-1 rule). You can see find my own, more philosophical discussion, of the topic here: https://docs.borgbase.com/strategy/. There I also distinguish between operational backups and archives. BorgBase is focused on the former. Offline backups are more suited for the latter.
It's not hard to create a cloud backup service where delete requires separate credentials which are not used in day-to-day operations (and so can be kept secure). And without these credentials a backup is kept N days and cannot be deleted or overwritten. Don't know if anyone do this, though.