Re: One could argue that I [and Bostrom, Rees, etc] are blowing the issue out of proportion. We have survived so far, right? (Wrong, actually - anthropic considerations indicate that survival so far is not evidence that we will survive for a lot longer, and technological progress indicates that risks in the future are worse than risks in the past).

Existence is not evidence, but the absence of previous large-scale disasters should certainly count for something. We have no evidence of civilisation previously arising and then collapsing, which we would expect to see if civilisation was fragile.