Years back (late 1990s) I worked with some mainframe people. They bragged that everything is hot fixable on the fly so they can apply security updates or replace broken hardware without rebooting. Then they admitted they schedule a reboot every 6 months anyway. Turns out the redundant backup power supply failed in at the same time and one hot patch was not applied to startup scripts and it took a week to figure out what was missing so the system booted again. By rebooting every 6 months they remember everything and so can get the system back up.

I probably have some details wrong in the story above. I worked with those people, but never on the mainframe. I think the point stands though, if you don't do something often it can't be done.