Isn't curing polio, etc a requirement for ensuring the survival of the species? What I mean is: doesn't it matter that we try to save each person rather than just a person?
In the same way, if a large portion of the population was suddenly killed today, in the long run it wouldn't affect the number of people. The total population limit/equilibrium is determined by food production, tech and social habits; but actual mass deaths (say, as ww1/ww2) are just a short term disturbance in the number of people and quickly get corrected.
It doesn't mean that curing polio is useless - it makes many people much happier; but simply there is no direct relation between happiness of one generation and wellbeing of non-immediate future generations. Culture gets passed on; built infrastructure gets passed on; reseach gets passed on; but there's almost no difference caused by generation suffering horrible conditions/war/torture [assuming no large permanent loss of knowledge/infrastructure; just the suffering&deaths] and that generation living happily. In the short term there's an effect, sure, but in the long term?
Is Germany suffering now in any way because the 30-year-war in 17th century killed 30-40% of the population? Would it be any better place if that war&suffering hadn't happened ? Definitely no. The lost people recover in a generation or two, and that's it.