We don't think AWS does periodic offline backups of stuff stored in S3 so that they don't find themselves in this exact embarrassing scenario? Regardless of whether it's user-facing for AWS users, I'd hope they do, or certainly that they did before they got a good sense of the long-term reliability of S3 as a big system.

I think they do all sorts of backup stuff - and as a consumer of S3 you need to evaluate how resilient their backing up is. S3 is pretty murky about permanence guarantees so I'd always look at making sure there is a separately maintained replication script to some medium I have control over if that data is irreplaceable and the costs of losing it are significant to the business.

Judging those costs and making that call is a complex matter of course.

What would such a service then do as an end-user beyond what `aws s3 sync s3://my/bucket /mnt/my/backup/drive/` does?

(Honest question, not provocation. To me either you trust AWS or any party to never lose your data [unwise], or you basically ask them to offer a way to rsync, which AWS does)

I think that's essentially what such a service would do. You might throw in some periodic less frequent syncs (maybe you sync down to the main backup every day and sync that backup to a secondary backup weekly or monthly) and maybe some of those successive syncs are done to hosts that are usually disconnected from the network to add in a firebreak.

They publish durability numbers for S3:

designed to provide 99.999999999% durability of objects over a given year

Well, versioned bucket replication is essentially backup when you configure it in a way that the writer to the first bucket can't do any operations on the second bucket. Since bucket replication doesn't replicate deletion markers you essentially end up with your data being duplicated instead of just replicated writes.