Another approach to justifying a low prior would be to say, “if such cost-effective strategies had been available, they would have been used up by now,” like the proverbial $20 bill lying on the ground. (Here, it’s a 20-util bill, which involves altruistic rather than egoistic incentives, but the point is still relevant.) Karnofsky has previously argued something similar.

Doesn't that justify a low prior expectation for marginal benefits of marginal investment in all charities?