[fulafel]

The parity cannot be used in the degraded scenario that was under discussion.

See eg here where the increasing disk size vs specified unrecoverable read error rate is explored in relation to the question at hand: https://queue.acm.org/detail.cfm?id=1670144 (in the article Adam Leventhal from Sun, the makers of ZFS, talks about the need for triple parity).

Also, the conclusion "ensure your backups are really working" is an important point irrespective of this question, since you'll also risk losing data due to buggy software, human errors, ransomware, etc.