In practice, when I seen a chance to do high return work on other x-risks, such as synthetic bio, I do such work. It can't always be done publicly though. It doesn't seem likely at all to me that UFAI isn't a solvable problem, given enough capable people working hard on it for a couple decades, and at the margin it's by far the least well funded major x-risk, so the real question, IMHO, is simply what organization has the best chance of actually turning funds into a solution. SIAI, FHI or build your own org, but saying it's impossible without checking is just being lazy/stingy, and is particularly non-credible from someone who isn't making a serious effort on any other x-risk either.