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.
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.