If safety was paramount no passenger plane would ever leave the ground, they do crash occasionally. Having an engineering background on critical machinery I would say everything has a use by date and having a lower limit of safety on cargo planes seems perfectly correct to me. After doing the flight between Singapore and London many times and seeing the hiding the 747's were given I would say that retiring them to the paddock carrying cargo is a very sensible thing to do. I would hate to think I was on a high hour high cycle 747-400 doing a long haul flight without an empty seat and full of cargo.