I didn't
make any argument -- at least, not any argument for or against AI x-risk. I am not, and was not, arguing (1) that AI does or doesn't in fact pose substantial existential risk, or (2) that we should or shouldn't put substantial resources into mitigating such risks.
I'm talking one meta-level up: if this sort of risk were a real problem, would all the arguments for worrying about it be dismissable as "hypothetical arguments"?
It looks to me as if the answer is yes. Maybe you're OK with that, maybe not.
(But yes, my meta-level argument is a "hypothetical argument" in the sense that it involves considering a possible way the world could be and asking what would happen then. If you consider that a problem, well, then I think you're terribly confused. There's nothing wrong with arguments of that form as such.)
The comparisons with extraterrestrials, religion, etc., are interesting. It seems to me that:
(1) In worlds where potentially-hostile aliens are listening for radio transmissions and will kill us if they detect them, I agree that probably usually we don't get any evidence of that until it's too late. (A bit like the alleged situation with AI x-risk.) I don't agree that this means we should assume that there is no danger; I think it means that ideally we would have tried to estimate whether there was any danger before starting to make a lot of radio transmissions. I think that if we had tried to estimate that we'd have decided the danger was very small, because there's no obvious reason why aliens with such power would wipe out every species they find. (And because if there are super-aggressive super-powerful aliens out there, we may well be screwed anyway.)
(2) If hell were real then we would expect to see evidence, which is one reason why I think the god of traditional Christianity is probably not real.
(3) As for yeti, cold fusion, etc., so far as I know no one is claiming anything like x-risk from these. The nearest analogue of AI x-risk claims for these (I think) would be, when the possibility was first raised, "this is interesting and worth a bit of effort to look into", which seems perfectly correct to me. We don't put much effort into searching for yeti or cold fusion now because people have looked in ways we'd expect to have found evidence, and not found the evidence. (That would be like not worrying about AI x-risk if we'd already built AI much smarter than us and nothing bad had happened.)