One advantage of multiplying is it shows it is helpful to cure diseases earlier, because the benefit is multiplied by the number of years of life remaining.
If you instead subtracted, it seems like we would have little reason not to delay treatment—because curing a 1yo and a 50yo of blindness would both count for the same number of years, implicitly valuing 49 years of sight at zero.
One advantage of multiplying is it shows it is helpful to cure diseases earlier, because the benefit is multiplied by the number of years of life remaining.
If you instead subtracted, it seems like we would have little reason not to delay treatment—because curing a 1yo and a 50yo of blindness would both count for the same number of years, implicitly valuing 49 years of sight at zero.