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There needs to be a DBA version of the saying, "RAID is not a backup"

Just expand RAID to "Replicas At Independent Datacenters".

While I totally agree here, replication/raid vs backups, I must say that having some weak (in terms of HW resources) replica somewhere in the closet is much much better than system with just single master.

It's not, but they complement each other nicely.

Which is what you must do if minimizing downtime is critical.

And, of course, your disaster recovery plan is incomplete until you've tested it (at scale). You don't want to be looking up Postgres documentation when you need to restore from a cold backup, you want to be following the checklist you have in your recovery plan and already verified.

Sure, but there are lots of failure modes where the failure goes with the streaming replication and all instances are trashed.