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This certainly can happen. You can't rely on being able to launch enough replacement instances if a zone fails.

You can at least mitigate it by keeping utilization of your instances below what would be needed to handle load of one zone failing. I suspect to save money, many people are pushing utilization as high as possible.

This is what reserved instances are for. You can by low use reserved instances for disaster planning.

If an outage in one zone causes a brownout in the other zones, then the aren't independent "availability" zones in any logical sense. They may as well be on the same power supply.

They are independent IF you buy them in advance. If you wait till an actual outage then it's too late.