Working in professional services a number of our clients still use AS/400 mainframe environments, with data migrations to other environments becoming more and more common.
I worked on a data migration from an AS/400 environment to an SQL DB. I kid you not, the database schema for the AS/400 environment was printed on old A3 scrolls that looked tea stained.
For simply that reason, they work and rarely have any issues. The problem is that those with the expertise to manage them and fix them in the event that things do go wrong are diminishing. And that's a big risk for businesses.
I wouldn't be given the people that use ours, day and night shifts, have never seen it crash or freeze for the decade or so they worked here. They say it only goes down when a cheap sysadmin accidentally unplugs it trying to swap out a different server or if they have to image stuff after suspected hacks. Otherwise, they're the Energizer bunny of servers.