Even when polio was at its worst it never threatened the survival of the species. Curing polio would save many people's lives, but it would have minimal impact on the survival of the species.

Think of it as this way: curing polio means more Einsteins for the future.

Actually, no. If right now 50% of kids started to die early, we'd have pretty much the same number of kids growing up to be Einsteins; we'd just compensate it by more births just as we have always done.

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.

Kitchen table physics is done. We already have universities brimming with underemployed PhDs producing many more exotic theories than we have the equipment to test in their lifetimes (assuming we can ever do so near Earth). More brains are no longer very useful when each requires a huge commitment of resources that we aren't providing.

Why then don't geniuses come from third world countries. Improved standard of living will help that, not pure numbers.

It's important to note that people struck by polio tend to be from the lower socioeconomic strata of developing countries. Not only is preventing polio important for saving individuals but it's also important from exacerbating the already terrible level of inequality across the world. It's an investment in both the present and the future.