TIL that many large banks, airlines, insurance companies, and government agencies are still using IBM mainframes, a ~50 year old computing platform. In fact mainframes still seem to be in active development: IBM announced its new z15 model last year.

Why are these ancient machines still around and widely used? One HackerNews user suggests high switching costs:

A mainframe is not a supercomputer. Science is done on clusters of x86 chips and Nvidia gpus with custom interconnect and Linux.
A system that runs a bank or an insurance company and wants good availability can be built in one of two ways: you either spend money on software that deals with the hardware being unreliable and save money on software (pioneered by Google) or you spend money on hardware that promises to be highly reliable and save on software.
No new player believes mainframe (extremely expensive hardware) is cost effective, they all use commodity hardware.
A bank that needs to run binaries from the 70s for which they don't have the source code can keep paying IBM and not investing in reverse engineering the binary and implementing it in Java.
A bank that has a billion lines of code in Cobol can compile it to run on jvm on commodity hardware and run it in parallel with the mainframe for a year to validate and then switch over to the new system and stop overpaying for hardware, but that sounds risky, so they keep paying a million dollars for a system with the same performance as a 50 thousand dollar server.

You can buy your own mainframe second hand, it seems to cost around $25k not including shipping. Oh, and shipping will be really expensive too, because the machine weights 700 kg.