Worth noting, in the 90's general computing was still increasing at a very rapid pace, where you could replace a late 80's mainframe with an early 90's x86 or PPC server. You won't have the built in layers of redundancy, but that isn't always necessary for any given environment. The cases of many mainframes still in use today are on much faster/newer systems where that critical uptime is much more important.

I'm not really a fan of mainframes, only pointing out that the companies still on them are generally on them as a direct replacement to general purpose servers isn't purpose fit, and conversion is excessively costly, even compared to IBM mainframe contracts.