One reason HP kept MPE around so long is backwards compatability. I don't think many people (certainly not me until I got into it) realize how often stuff is used because "it's working, why change it?" - to the extent of keeping 20 year old software & hardware running an operation because it works.
Sometimes, though, they do want the latest & greatest OS for security reasons, even if the HW & SW are unchanged. Strange as it sounds, it happens.