I assure you it's not the hardware that's keeping those systems alive, it's the cost and risk of rewriting the software. You have to find a consulting group willing and able to hire people capable of reading and understanding the outdated languages and converting to a new platform. And there are no test suites in place to help confirm the proper functioning of what is written.
My first programming job out of college in the late 80s was working on small mainframes with a custom accounting software package, and there are still several customers that use that system.