If I knew that I'd die painlessly while asleep and that my behaviorally-identical duplicate would seamlessly take over my body in the morning, that's about 95%

If I knew that I'd die painlessly while asleep and that my behaviorally-identical duplicate would seamlessly take over my body in the morning, that's about 95% of my worries-about-death already taken care of. I don't fear nonexistence (much). I fear pain and disability, and I worry about the effect of my death on other people. Gradual-replacement-by-machine would solve those problems nicely. Likewise, you'd get more shrug than horror out of me if I were told that my sister isn't really my sister, "merely" an entity with the same appearance, memories, history, and behavior as my sister.