C'mon guys. It's been 65 million years since the last devastating asteroid and nearly 100 years since that dinky little Tongusta strike. While it's possible that the next impact could be later tonight (that's OK, I've got some Lagavulin), chances are that we've probably got some time. Our detection abilities have improved radically in just the last dozen years or so, and will get better by the year. It seems likely to me that within a very few decades we should be able to plant ion drives or maybe just drop dark or light colored powder to take advantages of solar effects. With enough lead time, a light and consistent thrust should be more than enough to change an orbit without all the fireworks. In the meantime, we're doing quite enough to screw ourselves out of a habitable planet without even thinking about asteroids. -mc
In this case, the results are not devastating. Even those who have studied this (not me) say that it is not a world ending activity. The food chain will remain.
Unless you're near ground zero! I think "world ending" needs to be more clearly defined. It will be pretty darned devastating on a large scale, and it is fair to say that life for many will be drastically altered- not for the better. Many institutions of civilization will certainly suffer drastic changes. Standards of living even for those far from the impact area will be affected.
It seems to me you are deliberately downplaying the negative side of a mountain-sized impactor.