re: US Nuke Program computers run on 8" floppy disks, with code from the 50s

Plants do this.
In their case, there's a lot of old folks working them. They'd have to be retrained if the system was updated and modernized. Not to mention the fact that units in a plant need constant monitoring. If you were to upgrade, you'd lost the ability to monitor temperature, pressure, etc. which would be incredibly dangerous in a plant. So they'd have to shut down... Which would make them lose millions of dollars.

Whether or not the government does this for a similar reason is beyond me.