The arguments by Bostrom, Yudkowsky and others can be summarised as follows:
- Superintelligence is possible
- We don't know how to align a superintelligence
- An unaligned superintelligence could be catastrophically dangerous
I'm not sure if premise 1 is falsifiable, but it is provable. If someone either develops an AI with greater intelligence than a human, or discovers an alien with same, or provides proof through information theory or other scientific knowledge that greater-than-human intelligence is possible, then premise 1 is proven. (Someone more qualified than me: is this already proven?)
Premise 2 is falsifiable: if you can prove that some method will safely align a superintelligence then you have disproved the claim. To date, no one understands intelligence well enough to come up with such a proof, despite a lot of effort by people like Yudkowski, but the claim is not unfalsifiable in principle.
Admittedly premise 3 is less falsifiable, because it's a claim about risk (an unaligned superintelligence could be very dangerous, not definitely 100% will be). But to disagree with premise 3 you have to believe that an unaligned super-intelligence is definitely safe. Either you claim that no superintelligence of any alignment will ever be dangerous or you claim that humanity will always be able to restrain a rogue superintelligence. Neither of those are the sort of claim you could reasonably consider to be 100% certain.
At this point, we're down to debates about how large the risk is, and IMO that explains why Yudkowsky and Bostrom give lots of different scenarios, as a counter-argument to people who want to assume that only certain narrow paths lead to catastrophe.
I think me using the term "valid" was a very poor choice and saying "worth considering" was confusing. I agree that how you act on your beliefs/evidence should be down to the maximum expected utility and I think this is where the problems lie.
Definition below taken from Artificial Intelligence: A Modern Approach by Russell and Norvig.
If we use this definition what would we fill in to be the utility of the outcome of going extinct? Probably something like U(extinct)=0; the associated action might be something like not doing anything about AI alignment in this case. What would be enough (counter)evidence such that the action following from the principle of MEU would be to 'risk' the extinction? Unless I just overlooked something, I believe that e has to be 0 which is, as you said, not a probability in Bayesian probability theory. I hope this makes it more clear what I was trying to get at.
Your example of disjunctive style argument is very helpful. I guess you would state that none of them are 100% 'proof' of the earth being round but add (varying degrees of) probability to that hypothesis being true. That would mean that there is some very small probability that it might be flat. So then we would, with above expected utility function, never fly an airplane with associated actions for a flat earth as we would deem it very likely to crash and burn.
I would add to your last creationist point