SF author Robert Sawyer was recently asked a question about this: "What's your opinion of Fermi Paradox?"
Sawyer's reply: "Enrico Fermi raised a very valid point: if the universe should be teeming with life, then where are all the aliens? My fear is that terrorism ultimately destroys all civilizations: that is, as more and more destructive power ends up in the hands of individuals, someone eventually does enough damage to destroy each civilization."
So, yesterday it was nuclear Armageddon that was the ultimate crisis, but today it's terrorism? I can't help but be amused by Sawyer's reply.
Now that we've survived the Cold War, the foremost security crisis that shaped our worldview yesterday, we've moved on to suppose that the real danger to species generally is the foremost security crisis shaping our worldview today?!? Please. Paranoid shortsighted rubbish. But trendy, very trendy.
Asteroids are demonstrably just as dangerous to the survival of our civilization as terrorism is, if not more so. Just ask the dinosaurs.
Frankly, living is a dangerous business, and thinking and acting intelligently is hard. If extinctions of intelligent species are common, why does it have to predominately be from only one cause?
One could pick any of the man-made threats we face today and reasonably suppose that somewhere there could be alien species succumbing to those problems: economic collapse, genocide, terrorism, militarization, pollution, and so on.
That's not to mention the potential for both terrestrial and extraterrestrial natural disasters. Plague, asteroids, and radiation are only some of the possibilities.
Let's not forget alien invasion, which if it happened would tend to be much more one-sided than is generally depicted in popular science fiction. Perhaps there is an alien race with FTL going around and snuffing out all the species that announce their presence with radio waves, as if they are pulling weeds out of their garden.
Or, maybe we're just among the first. Why should we expect aliens to have already done what we just accomplished within the last one hundred years?