Quick semantic cleanup: Redis has two types of persistence: snapshot persistence (RDB) and journal persistence (AOF). It sounds like they tried only snapshot ("copy the world") persistence, found it to increase latency, then turned it off. It probably would have been okay with journal persistence.
Their setup would have been fine _if_ the master never came back up. If the master didn't restart, then something in their infrastructure would have probably started using the replicas for data (and/or they would have promoted one of the replicas to be the new master, then the other replica would replicate from the new master).
But—having persistence on the replicas did allow them to copy the dataset off to backup storage. Then they were able to restore the old data when it was needed. So, we can prove their setup was persistent since they lost all their data and recovered it. :)
Their setup would have been fine _if_ the master never came back up. If the master didn't restart, then something in their infrastructure would have probably started using the replicas for data (and/or they would have promoted one of the replicas to be the new master, then the other replica would replicate from the new master).
But—having persistence on the replicas did allow them to copy the dataset off to backup storage. Then they were able to restore the old data when it was needed. So, we can prove their setup was persistent since they lost all their data and recovered it. :)