On a very large scale, if you think FAI stands a serious chance of saving the world, then humanity should dump a bunch of effort into it, and if nobody's dumping effort into it then you should dump more effort than currently into it. Calculations of marginal impact in POKO/dollar are sensible for comparing two x-risk mitigation efforts in demand of money, but in this case each marginal added dollar is rightly going to account for a very tiny slice of probability, and this is not Pascal's Wager. Large efforts with a success-or-failure criterion are rightly, justly, and unavoidably going to end up with small marginal probabilities per added unit effort. It would only be Pascal's Wager if the whole route-to-humanity-being-OK were assigned a tiny probability, and then a large payoff used to shut down further discussion of whether the next unit of effort should go there or to a different x-risk.

Eliezer explains it in his other comment (emphasis mine):

Before scanning, I precommit to renouncing, abjuring, and distancing MIRI from the argument in the video if it argues for no probability higher than 1 in 2000 of FAI saving the world, because I myself do not positively engage in long-term projects on the basis of probabilities that low (though I sometimes avoid doing things for dangers that small). There ought to be at least one x-risk effort with a greater probability of saving the world than this - or if not, you ought to make one. If you know yourself for an NPC and that you cannot start such a project yourself, you ought to throw money at anyone launching a new project whose probability of saving the world is not known to be this small.