You have to pay 2x for multi-AZ or you get downtime for upgrades. And DB major version upgrades require manual effort unless you want to roll the dice on their new blue-green feature, which can take hours to fail or finish cutting over.

Given that the full AWS setup that would replace that one server would cost closer to $6000-8000 / month, they could just use that money to buy a bunch of extra hard drives, a backup server, and hire a junior dev/sysadmin whose only job is to watch over it, still coming out ahead of AWS.