[I]n the 1960s, the most cost-effective charity was childhood vaccinations, but now so many people have donated to this cause that 80% of children are vaccinated and the remainder are unreachable for really good reasons (like they’re in violent tribal areas of Afghanistan or something) and not just because no one wants to pay for them. In the 1960s, iodizing salt might have been the highest-utility intervention, but now most of the low-iodine areas have been identified and corrected. While there is still much to be done, we have run out of interventions quite as easy and cost-effective as those. And one day, God willing, we will end malaria and maybe we will never see a charity as effective as the Against Malaria [Foundation] again.
I don’t have a direct source on the argument that you said Elie Hassenfeld made, but I do have a quote from Scott Alexander (http://slatestarcodex.com/2013/04/05/investment-and-inefficient-charity/) who went to a live event in which Elie made this argument:
Thanks! I’ve edited the post to include a link to that article.