"rotors always need to be changed when pads are gone " - perhaps if you keep driving after the pads are gone.  You will destroy the rotors very quickly if the pads are totally gone.  While this seems obvious to all of us there may be some drivers who simple drive with 'extra Noise' for a few extra days and grind down the metal rotors.  Regardless - if the pads never wear down to the rivets and the brakes don't see abuse (Porsche brakes are way overbuilt and hard to abuse on the street at normal speed) the rotors could easily last the life of the car.  The only service I ever did on the PCCB was brake fliud change once after seven years.