Do you know about worst case analysis?

Let's say your data loss occurs on the Sunday, 31th of a month at 23:55 and then gets synched across all your S3 backups (or it could occur at some point before that but you don't notice it). And poof goes your data.

You could try adding objects with the date in their names to a 'backup' bucket rather than create separate buckets for each backup. S3 also supports object expiration, so you can set your daily backups to expire after 7 days and weekly backups to expire after 4 weeks, for example.