Running on a single server does not mean no backups and uptime monitoring is in place. If the hardware fails, you get a ping on the channel of your choice, manually provision a new VM then download the latest sqlite backup from your backup provider. Easy to make a checklist or a script for this, too.

Also there is a third option between HA (with its increased cost and complexity) and "we don't upgrade if it's working, because it could break", which is "take the site down for a few minutes, do the upgrade, bring it back up". It's not for every site, but there's a range of sites for which that is fine.