I don’t think the post is correct in concluding that the current marginal cost-per-life-saved estimates are wrong. Annual malaria deaths are around 450k, and if you gave the Against Malaria Foundation $5k * 450k ($2.3B) they would not be able to make sure no one died from malaria in 2020, but still wouldn’t give much evidence that $5k was too low an estimate for the marginal cost. It just means that AMF would have lots of difficulty scaling up so much, that some deaths can’t be prevented by distributing nets, that some places are harder to work in, etc.
It does mean that big funders have seen the current cost-per-life saved numbers and decided not to give those organizations all the money they’d be able to use at that cost-effectiveness. But there are lots of reasons other than what Ben gives for why you might decide to do that, including:
You have multiple things you care about and are following a strategy of funding each of them some. For example, OpenPhil has also funded animal charities and existential risk reduction.
You don’t want a dynamic where you’re responsible for the vast majority of a supposedly independent organization’s funding.
You think better giving opportunities may become available in the future and want to have funds if that happens.