You're assumption is that you get a fixed amount of functional life, followed by a variable amount of less- or non-functional life. OTOH, you might get a varia

You're assumption is that you get a fixed amount of functional life, followed by a variable amount of less- or non-functional life. OTOH, you might get a variable amount of functional life, followed by a more-or-less fixed amount of non-functional life. In general, I think the second option more closely matches how we understand aging and disease. (Also, watch the TED talk by the referenced professor. He doesn't ignore quality of life.)