Centurion lounges definitely exist - in South and Central America I saw a few and they were useful even if you have PPass as they are usually land side and they are happy with a flight that day in general - ie. can use them for both departures and arrivals and I'm sure you could get away with saying you were flying later even if you weren't flying that day at all. With a cent card you can also take free guests unlike with PPass lounges (not sure on this for plat cards).