You're hypothesising about something that has never happened. Check out some 3rd party cloud uptime metrics - the major providers (AWS, Google, Azure) have had less than an hour of downtime in the past year. Reliability is no longer on the agenda - it has been proven.
It did happen and my clients were affected. After AWS fu$%up rollout of software update in 2011 that overvelmed their control plane and had whole zones down for many hours and took many days to fully restore, they rolled out patches that throttle cross zone migration. After those patches at one point Netflix was having issues and started massive migration that hit throttle thresholds and affected ability of other tenants to move to non affected zones. It's very far from hypothetical given netflix consumes about 30% of resources (which translates to many whole size physical datacenters) if they spike 50% they will overvelm the spare capacity.
