> And to me, a society in which the life expectancy at birth is 75 yrs (with a modest deviation from the mean) is clearly better to live in than one where the life expectancy is 25 yrs at birth. In that sense I still think "ancestors were dumb" because they didn't know what bacteria was and that it could kill you, for example.

The key phrase is "at birth". The vast majority of the difference between "75 years at birth" and "25 years at birth" is due to infant/toddler mortality.

If you made it to 10, your life expectancy wasn't a lot different than today.