“[To conserve scarce resources, there is a] responsibility to end one’s life in the absence of any terminal illness . . . a duty to die even when one would prefer to live. . . . A duty to die is more likely when you have already lived a rich and full life. You have already had a full share of the good things life offers. . . . To have reached the age of, say 75 or 80 years without being ready to die is itself a moral failing, the sign of a life out of touch with life’s basic realities.”