I don't know if this is a crazy idea or if it creates scalability issues, but could you craft an algorithm to cold store data for users who do not show a need for instant access, and/or warm up the data when you predict it will be needed? Kind of like a physical logistics company would need to do with distributed warehousing.

Sticking points I see are, 1. If you get it wrong you'll need some form of UX that keeps the users from getting to angry about it. 2. The cost of moving the data between hot/cold storage might make this prohibitive until a much larger scale. 3. User behaviors might not be predictable enough.

So from a completely evil (well, capitalist) perspective, do you have data on how often people retrieve backups, and at what 'age' they do so?

Because there may be an inflection point that offering monetary compensation for data loss, rather than actually trying to store the data, would make more financial sense. I.e., "All data > than 2 years gets silently expunged, and anyone trying to retrieve it at that point gets $10 per gig in compensation for 'our mistake'".

Please don't actually consider that though.