Just because a piece of software is old, doesn't mean it's suddenly doesn't do its intended function.
I'm not sure I'd be shocked by the effort that people make to keep old software running,. You mention PDP emulators, but how many people here use DOSBox on a regular basis to play old games.
Emulators are just one way do keep old software running of. The other way if the source is around is to keep updating the software for new platforms but avoiding too much feeping creaturism if you can. That's pretty much where I'm at with doing ircd work, keep the code updated for modern systems(with their own OS specific quirks) so it continues function.
It seems like people just want new and shiny software just for the sake of having new and shiny. New and shiny code however doesn't have X number of years of being used as production and all of the WTF bugs have long been squashed.