Approximately everyone (at least in the F500) outside of Big Tech uses them. For example, Costco's entire inventory management system runs on IBM i (so, POWER). You can see the classic terminal look around the store. Banks run a TON of z and i. You'll never see them because they're essentially always in data centers, but I guarantee you interact with them even if it's very non-obvious because there's 50 microservices between the UI and the actual system of record.