I'm not sure I understand the use case you mention. Is it in essence the idea of someone abusing an unlimited plan to back up more data that you'd reasonably expect them to? Can it be boiled down to Backblaze's cost of hard drive space and whether users pay enough to cover Backblaze's costs?

The use case you mention seems to be very exotic, but who knows, maybe it's not as rare as I would expect. To me, expunging deleted files because of this rare, somewhat-malicious case seems like it would cause undesirable collateral damage among your service's typical users.