Duct tape and bubble gum?
20 year old (and older systems) often won't be changed for a very simple business principle
They just work and there is 20 plus years of empirical evidence to support that.
No duct tape and bubble gum - these things are much more likely to be belt and braces systems than a lot of the crap being pumped out by modern software houses where "responsive to business needs" means "we'll be wanting some more money from you next year to fix this pile of steaming shite that we just sold you".