> I've not actually seen an AZ go down in isolation
Counterpoint: I have. Maybe not completely down, but degraded, or out of capacity on an instance type, or some other silly issue that caused an AZ drain. It happens.
While I agree, I remember we once had cross-region replication for some product but when AWS was down the service was down anyway because of some dependency. Things were working fine during our DR exercises, but when the actual failure arrived, cross-region turned out useless.
Counterpoint: I have. Maybe not completely down, but degraded, or out of capacity on an instance type, or some other silly issue that caused an AZ drain. It happens.