At a previous employer, we met with AWS engineers who told us that this sort of thinking wasn't correct. Availability zones are for failure tolerance. Regions are conceptually different and for things like moving compute closer to clients, mitigating political instability or ensuring that data is held in a certain jurisdiction. To back up that claim, they told us that amazon.com ran out of a single region, us-west-2.
As an aside, another interesting point from that meeting was that autoscaling wasn't a good strategy for scaling up in the event of the loss of an availability zone. The reason why is that in those scenarios, a lot of other customers will be trying to fail over as well and autoscaling can get very slow. Instead of autoscaling, they recommended that we run 150% of our per-AZ capacity in 3 AZs so that if an AZ went offline, the remaining two would already have the necessary capacity. We still used autoscaling, but it was for responding to spikes and troughs in our own load, not failure scenarios.
I'm curious whether these two observations are correct since it seems like a lot of people view regions and autoscaling through a failure-recovery lens.