I think that the reason we don't use average lifespan as our measuring stick is because people are far more bothered by the variability in that metric than its mean. If 20 year olds were offered the choice of "do you want the average life expectancy or do you want a rock-solid guarantee that you will achieve average minus 5", I suspect that most people choose the guarantee. Not just because of hyperbolic discounting, but also because the average 73 year old has accomplished what they plan to accomplish in life. Whereas people are very afraid to die while they have small children, before they've proven themselves in their careers and before they've experienced retirement.
I think that the reason we don't use average lifespan as our measuring stick is because people are far more bothered by the variability in that metric than its mean. If 20 year olds were offered the choice of "do you want the average life expectancy or do you want a rock-solid guarantee that you will achieve average minus 5", I suspect that most people choose the guarantee. Not just because of hyperbolic discounting, but also because the average 73 year old has accomplished what they plan to accomplish in life. Whereas people are very afraid to die while they have small children, before they've proven themselves in their careers and before they've experienced retirement.