I really hate that argument as a reason that we have to explore NOW
The idea that danger in hundreds of thousands or millions of years should determine our exploration schedule over the next century is crazy.
Gary has come up with the only coherent version of the argument, saying that if we don't advance in space now, we'll simply give up and never try again, and next thing you know, a billion years will have passed and the sun will boil away the oceans. As you might guess, I think it's a coherent argument but a wrong one.