Earlier quoted context omitted.
If you retrieve the data slowly it's cheap. It's expensive if say you are a retailer who needs their database restored asap. I use a Linux perl client!
I suppose the biggest risk is failure-to-pay, as with all cloud backups/storage. If I allow my payment card to expire, Amazon aren't obligated to continue to store my data. If I drop off the grid for an extended holiday, that could be a real risk. To my knowledge, Amazon offer no means of prepaying.
Business idea: cloud archive storage where you pay when you upload data and optionally pay a modest monthly fee for real-time access to stored data, but they'll guarantee to keep your data for you if you stop paying: you'll just need to pay to retrieve that data.
As the long-term archival data wouldn't need to be stored in a data-center: just a commodity tape-library box in a basement in a farm somewhere near a freeway I imagine it would be kinda cheap to run as a business. You could set-up a Foundation or other entity to ensure long-term continuity of operations and have it self-sufficient through an endowment. E.g. a $1m endowment would easily pay for something like this into perpetuity.