I think that resignation is something that may be more frequent in the future.
People simply live much longer these days than in years past. I have read that of the 266 Catholic popes (from St Peter to Pope Benedict), Pope Benedict (at 85 years old) is the 4th oldest throughout history. In centuries past, there was not nearly the medical sophistication or technology to care for someone when they became old or ill.... so they died at an earlier age.
Also, the physical demands of the position are much greater in these modern times, where the pope is (practically) expected to travel frequently and be in frequent communication with all of the media. This just didn't happen even 50 or 100 years ago. In earlier times, the pope often spent much of his time in Rome, using written communication and local audiences to communicate with the world. I am quite certain that Pope John Paul II traveled more miles during his papacy than all 264 popes before him combined.... Perhaps even at least 10X more.
Popes are merely human, after all. Mind, body and spirit are needed in sufficient capacity to carry out their responsibilities.