When I used to work with US bank and financial institutions we used to do "disaster recovery tests" and it effectively meant they shut off a whole site and ran on their backups to see what happened.
Even if it was "only" 2x or 3x the expense to have a completely multizone setup, the ongoing operational burden is too high to justify over the 2, 10, or 100 year timeframe when no outages happen which could have been avoided.
Companies dependent on a single Amazon zone just give up and wait during a zone outage, blame it on Amazon, then get back to making money the next day.
Contrast with no AZ now or in the next quarter. Then surely people would take precautions like switching to Google or equinix at a minimum.
AT&T used to put telecom switches in bunkers.