Phil,

There are fairly quantifiable risks of human extinction, e.g. from dinosaur-killer asteroid impacts, for which there are clear paths to convert dollars to reduced extinction risk. If the probability of AI (or grey goo, or some other exotic risk) existential risks were low enough (neglecting the creation of hell-worlds with negative utility), then you could neglect in favor of those other risks. The argument that "I should cut back on certain precautions because X is even more reckless/evil/confused and the marginal increase in my chance of beating X outweighs the worse expected outcome of my project succeeding first" is not wrong, arms races are nasty, but it goes wrong when it is used in a biased fashion.