> I don't mean this in a condescending way at all, but really out of sheer curiosity: Who uses mainframes nowadays and what for?

There's probably some minor strategic relevance here. E.g. for the government which has some workloads (research labs, etc.) that suit these systems, it's probably a decent idea not to try and migrate over to differently-shaped compute just to keep this CPU IP and its dev teams alive at IBM, to make sure that the US stays capable of whipping up high-performance CPU core designs even if Intel/AMD/Apple falter.