The hunter-gatherer argument also fails an important sniff test for me. Evolutionarily-driven arguments fail to take into account that what may have been sufficient outcomes to drive selection (e.g. survival to/through reproductive prime) do not meet our current values and standards of acceptability. Heart disease, diabetes, cancer, etc. tend to strike after people are done reproducing but long before what we consider reasonable life expectancy.
Evolution can continue to exert pressure after you've passed reproductive age, if you stick around and help raise your offspring, which humans generally do.
You also hint at traditional societies having much shorter lifespans than modern ones; it's true they have much higher rates of infant and childhood mortality, but lifespans for people who make it past childhood are very similar.
I agree evolutionary arguments in and of themselves will never be convincing (since there will always necessarily be a large amount of conjecture), but they are not being made in isolation: they providing an explanation for the observed fact that people living in modern societies are suffering a rising epidemic of degenerative diseases, and people living in traditional societies are not.
See also: the fantastic book "Nutrition and Physical Degeneration" by Weston Price.
