Why should the backup land in S3, and not local somewhere?
Any good reason why one should pay for the additional storage and transfer costs?
Good question. The key point in my statement was "db of this size".
The problem with local backup is that space is not infinite.
Another problem is storage devices fail. S3 storage lakes _should_ be checking your data integrity on a regular basis and possibly maintaining copies of it iin multiple locations so you're not vulnerable to a site disaster.