[MaxBarraclough]

If offered by an organisation that I can trust to still exist in, say, 40 years time, this might work. Maybe Google/Amazon/Microsoft have this credibility. Maybe. There's no way it would work as a start-up.

You'd also need to charge enough upfront that you can still turn a profit if they stop paying immediately. Asking for upfront payment of 40 years of data-storage fees, might be a problem.

> a commodity tape-library box in a basement in a farm somewhere near a freeway

I'm not a data-storage expert by any means, but that doesn't sound anywhere near good enough. You need to redundantly protect against flood, fire, crime, etc. You'll also need to be able to retrieve data at scale. You're essentially rolling your own Amazon Glacier.