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.
And some of those i and z systems have some obscene engineering in them. I cannot find it now but i remember seeing a cross section of their 40 layer PCB simply so they could get more memory/IO performance to the CPU. POWER may have dropped out of the consumer space but it still has a vital place in servers.