For example, talking as if you think "simpler explanation" is a knockdown argument for evolution (which it is)
I don't quite agree - by itself, X being "simpler" is a reason to increase my subjective belief that X is true (assuming that X is in a field where simplicity generally works) but it's not enough to prove e.g. creationism false. Rather, it is the total lack of evidence for anything supernatural that is the knockdown argument - if I had reason to believe that even one instance of say, ghosts or the effects of prayer were true, then I'd have to think that creationism was possible as well.
I don't think investors actually believe there is zero risk - the phrase "risk-free" is a holdover from the assumptions of theoretical asset pricing models, and now everyone who gets trained in economics or finance is just being sloppy.
It would take more than the USA ceasing to exist for bonds to become worthless - maybe the USA will peacefully disincorporate, but still unwind all its debts. It would take something that would destroy the productive capacity of the USA as a whole, or something that causes massive tax revolts - an asteroid strike, or an alien invasion maybe? If you're an optimist about the economic effects of AI/nanotech etc, then it seems that should make the chance of defaults less likely, not more.
A calculated probability of 0.0000001 should diminish the emotional strength of any anticipation, positive or negative, by a factor of ten million.
I don't play the lottery, but I sometimes have pleasurable daydreams about what I'd do if I were some great success - found the cure for cancer, proved P=NP, won a Nobel prize... objectively speaking, the probability is extremely low, but it doesn't scale my pleasure down by a million times.
Silas: red is not the set, but what all of those things have in common. The set would be most effective if you presented a sequence of examples that was different in every way except in color. To be extra sure of getting the point across, you could present examples that are exactly the same, except in color, and then say one was "red" and the other was "not red" - a whole educational philosophy has been built up out of this (look up Siegfried Engelmann and Direct Instruction). Of course this method of communication assumes that the audience is sighted, not colorblind, understands the concept of "same" and "different", etc.