Funding SIAI is optimal only if you think that the pursuit of Friendly AI is by far the most important component of existential risk reduction

This seems to assume that existential risk reduction is the only thing people care about. I doubt I am the only person who wants more from the universe than eliminating risk of humans going extinct. I would trade increased chance of extinction for a commensurate change in the probable outcomes if we survive. Frankly I would consider it insane not to be willing make such a trade.