Right but this doesn't necessarily have to be a drive failure. (Pun not intended.) Imagine if there was some id collision due to a db migration or something, and then one user ends up deleting the other user's data accross all backups. Unless google never deletes any data, even when the user deletes it themselves, then you can't be saved from that.

There are many types of backups (e.g. replication, point-in-time replication, periodic incremental backups, periodic full backups) and not all of them project against the worst kind of issues, e.g. a software bug that accidentally deletes a whole bunch of data.