First, fourfathom, thank you. You said it better than I could. Those other events (war, famine...) are not extinction level events.
This thread has ran it course sometime ago, but I like to correct a misconception prior to burying this dead horse.
Migrating people off planet is not the only way to save our species in an asteroid strike. A better way is to avoid the strike. Lucky for us, it is actually rather difficult for an asteroid to hit Earth. Both the asteroid and the Earth have to be at that exact spot at that exact same time and that is hard.
Earth diameter is 12,700km and Earth orbits the sun at 30 km/s, that is, Earth moved by more than two Earth diameter every second! Earth moved it's diameter in just over 7 minutes. Asteroids take decades or centuries to reach us and if we delay or advance the asteroid hit by merely 1/2 second 7 minutes, Earth is not there for the asteroid to hit when the asteroid got there. The asteroid itself likely is traveling at least as fast as Earth, we can "add" the movement of the asteroid into the equation and that will give us a large leeway still.
Further more, asteroids come from afar. Two straight lines originating from the same point and going in the same direction, with the second line a small angle off the first; the father you go, the bigger the separation between the lines. Thus, small change in angle early will enlarged to become a big change after traveling a long distance. This is another factor the will help us. Small change early can have big effect on trajectory changing both the position of orbital intersection and the intersection time.
If we throw a rock in one direction, Newton's 3rd law: an equal force will push us in the opposite direction. This little push changes the asteroid trajectory. Trajectory change alters the intersection time and the location it will intersect with Earth orbit. One rock is one small push, but we can do a lot of small pushes and let the small effects accumulate. This is a way some astrophysicist proposed we can use to avoid collision. Throwing rocks and throwing lots of them is where spinlaunch, or sling launch, or rail launch can come into play.
Asteroids likely has some ice or other frozen gases. There adds another way: heat the gas to and vent the gas-stream to push the Asteroids like a rocket's gas-stream pushes the rocket.
Granted, moving a team of engineers to an asteroid to build nuclear power plant(s), melt frozen gases, implement an array of auto rock throwers, and do that while the asteroid is still far enough away is not an easy task and it will cost trillions. Being trillions poorer still beats extinction.
EDIT:
Correction made. I misread a comma as decimal. Earth diameter is 12,700 kilometer, not 12.700 kilometer. So I did the mental math and arrived to the wrong 1/2 second number. It should be about 7 minutes. And we will need a lock more rocks, but method still works, in theory. Thanks to AVGresponding for pointing it out, I appreciated it.