Comment by ndriscoll

Newer mainframes are still faster than older mainframes, and can have hundreds of cores and 10s of TB of RAM. A big part of IBM's draw is that they make modern systems that will continue to run your software forever with no modifications. I had an older guy there tell me a story about them changing a default in some ISPF panel, and customers complained enough that they had to change it back. Their storage systems have a virtualization layer for old programs that send commands to move the heads of a drive that hasn't been manufactured for 55 years or whatever and translate that to use storage backed by a modern RAID with normal disks. The engineers in the mainframe groups know who their customer base is and what they want.

It's unlikely that they're literally using 40 year old hardware since the replacement parts for that would be a nightmare to find and almost certainly more expensive than a compatible new machine.