I think you're missing the point; I actually do think NASA is one of the best organizations to handle anti-asteroid missions and nukes are a vital tool since the more gradual techniques may well take more time than we have.
Your application of cynicism proves everything, and so proves nothing. Every strategy can be - rightly - pointed out to benefit some group and disadvantage some other group.
The only time this wouldn't apply is if someone claiming a particular risk is higher than estimated and was doing absolutely nothing about it whatsoever and so couldn't benefit from attempts to address it. And in that case, one would be vastly more justified in discounting them because they themselves don't seem to actually believe it rather than believing them because this particular use of Outside View doesn't penalize them.
(Or to put it another more philosophical way: what sort of agent believes that X is a valuable problem to work on, and also doesn't believe that whatever Y approach he is taking is the best approach for him to be taking? One can of course believe that there are better approaches for other people - 'if I were a mathematical genius, I could be making more progress on FAI than if I were an ordinary person whose main skills are OK writing and research' - or for counterfactual selves with stronger willpower, but for oneself? This is analogous to Moore's paradox or the epistemic question, what sort of agent doesn't believe that his current beliefs are the best for him to hold? "It's raining outside, but I don't believe it is." So this leads to a remarkable result: for every agent which is trying to accomplish something, we can cynically say 'how very convenient that the approach you think is best is the one you happen to be using! How awfully awfully convenient! Not.' And since we can say it for every agent equally, the argument is entirely useless.)
Incidentally:
it does seem like FAI has a special attraction for armchair rationalists:
I think you badly overstate your case here. Most armchair rationalists seem to much prefer activities like... saving the world by debunking theism (again). How many issues have Skeptic or Skeptical Inquirer devoted to discussing FAI?
There's a much more obvious reason why many LWers would find FAI interesting other than the concept being some sort of attractive death spiral for armchair rationalists in general...
FHI, for what it's worth, does say that simulation shutdown is underestimated but doesn't suggest doing anything.
My friend, hearing me recount tales of LessWrong, recently asked me if I thought it was simply a coincidence that so many LessWrong rationality nerds cared so much about creating Friendly AI. "If Eliezer had simply been obsessed by saving the world from asteroids, would they all be focused on that?"
Obviously one possibility (the inside view) is simply that rationality compels you to focus on FAI. But if we take the outside view for a second, it does seem like FAI has a special attraction for armchair rationalists: it's the rare heroic act that can be accomplished without ever confronting reality.
After all, if you want to save the planet from an asteroid, you have to do a lot of work! You have to build stuff and test it and just generally solve a lot of gritty engineering problems. But if you want to save the planet from AI, you can conveniently do the whole thing without getting out of bed.
Indeed, as the Tool AI debate as shown, SIAI types have withdrawn from reality even further. There are a lot of AI researchers who spend a lot of time building models, analyzing data, and generally solving a lot of gritty engineering problems all day. But the SIAI view conveniently says this is all very dangerous and that one shouldn't even begin to try implementing anything like an AI until one has perfectly solved all of the theoretical problems first.
Obviously this isn't any sort of proof that working on FAI is irrational, but it does seem awfully suspicious that people who really like to spend their time thinking about ideas have managed to persuade themselves that they can save the entire species from certain doom just by thinking about ideas.