This one surprised me least of all. The issue with upgrading these systems to newer technology, especially on general purpose operating systems, is that they're far less reliable, precisely due to the fact that they are more powerful and flexible, and hence, more complex.

The first rule of critical systems is to reduce complexity, and their current systems probably found the sweet spot between useability and simplicity decades ago.

The guy using the IBM accounting machines though... Dear god man! It sounds like the museum would've handed him a new system on a platter, in exchange for his old system. Take the upgrade!