But if there's even a chance …

Holden cites two posts (Why We Can’t Take Expected Value Estimates Literally and Maximizing Cost-effectiveness via Critical Inquiry). They are supposed to support the argument that small or very small changes to the probability of an existential risk event occurring are not worth caring about or donating money towards.

I think that these posts both have serious problems (see the comments, esp Carl Shulman's). In particular Why We Can’t Take Expected Value Estimates Literally was heavily criticised by Robin Hanson in On Fudge Factors.

Robin Hanson has been listed as the other major "intelligent/competent" critic of SIAI. That he criticises what seems to be the keystone of Holden's argument should be cause for concern for Holden. (after all, if "even a chance" is good enough, then all the other criticisms melt away).

This would be a much more serious criticism of SIAI if Holden and Hanson could come to agreement on what exactly the problem with SIAI is, and if Holden could sort out the problems with these two supporting posts*

(*of course they won't do that without substantial revision of one or both of their positions because Hanson is on the same page as the rest of SIAI with regard to expected utility, see On Fudge Factors. Hanson's disagreement with SIAI is a different one; approximately that Hanson thinks ems first is likely and that a singleton is both bad and unlikely, and Hanson's axiology is significantly unintuitive to the extent that he is not really on the same page as most people with regard to what counts as a good or bad outcome)