The definition of LEV I used in the previous post is: “Longevity Escape Velocity (LEV) is the minimum rate of medical progress such that individual life expectancy is raised by at least one year per year if medical interventions are used”. So it doesn’t lead to an unbounded life expectancy. In fact, with a simplified calculation, in the first post I calculated life expectancy after LEV to be approximately 1000 years. 1000 years is what comes up using the same idea as your hydra example (risk of death flat at the risk of death of a young person), but in reality it should be slightly less, because in the calculation I left out the part when risk of death starts falling just after hitting LEV. We are not dealing with infinite utilities.
The main measure of impact I gave in the post comes from these three values and some corrections:
1000 QALYs: life expectancy of a person after hitting LEV
36,500,000 deaths/year due to aging
Expected number of years LEV is made closer by (by a given project examined)
Terence Tao even talked about this in his Google+ profile.