It seems to me that the premise of funding SI is that people smarter (or more appropriately specialized) than you will then be able to make discoveries that otherwise would be underfunded or wrongly-purposed.

But then SI has to have dramatically better idea what research has to be funded to protect the mankind, than every other group of people capable of either performing such research or employing people to perform such research.

Muehlhauser has stated that SI should be compared to alternatives in form of the organizations working on the AI risk mitigation, but that seems like an overly narrow choice reliant on presumption that it is not an alternative to not work on AI risk mitigation now.

For example, 100 years ago it would seem to have been too early to fund work on AI risk mitigation; that may still be the case; as the time gone on one could naturally expect that the opinions will form a distribution and the first organizations offering AI risk mitigation will pop up earlier than the time at which such work is effective. When we look into the past through the goggles of notoriety, we don't see all the failed early starts.