Interesting question.
I think a modern day Dinokiller destroys all human societies but humanity manages to survive.
I know there are projects deliberately looking for Dinokiller 2.0; I would not be surprised if a near miss scenario could be identified vast ages in advance in terms of an object that could potentially collide with Earth. The spirit of the scenario doesn’t seem to be some kind of millennia to stop the threat, but something like a year or less.
Trying to prepare for a scenario like this sounds utterly freakish; consider having to evacuate the whole state of Florida - where do they go? How do you convince them that they need to move to something like the Australian Outback or Central Africa? The political will to act to save lives is probably not there. By the time a slowly brightening celestial body is visible in the skies, there will be only days to act.
Throwing nuclear weapons at something of this size could potentially vaporize small pieces of it, but a gigaton of nuclear weapons would change the specifics by less than a rounding error. Ideas like painting the asteroid, strap on parachutes or otherwise change its course have chances. But we have to run the premise that the impact happens.
There is a global reverse-911 call, but a lot of people either didn’t believe this would happen or are woefully unprepared. There’s enough time for people to try to protect from the blast, but nowhere enough time (or political will) to stockpile enough food to survive.
The initial deaths from the blast are soon overwhelmed by the collapse of food supply. Earth doesn’t have a year of food for its billions of people lying around; it might take a whole decade to accumulate such. Rationing in the pre-Dinokiller days would not have added much, and the massive fires that raze most of the United States and Latin America also torch the Great Plains.
It is this lack of food that is the largest killer after this disaster. It’s plausible to suggest that Switzerland survives the initial impact with minimal loss of life, owing to endless bunkers and strongpoints. But this is Carl Sagan’s nuclear winter scenario made real, and a decade of starvation awaits.
There’s a large difference between there being NO FOOD and NOT ENOUGH FOOD, with the terrible grim clarity. The spiral into societal collapse has military units turn into bandit gangs, funerals into cannibalistic feasts, and culture being replaced with the overarching need to survive. This is how human society ends–under the incredible trauma of starvation and a need to survive by killing others, the humanity that sees a dawn ten years later is immediately alien to us.
Cultures that are harsh tend to be a reflection of cruel times. There is no waking up from this nightmare; instead, the children born into this world will understand that this is reality and the strange ruins that still dot the world speak to an mythical state of civilization. The longest lasting legacy of Dinokiller 2, beyond even cultural consequences that take centuries to fade into myth, is the depopulation of humanity. The bottleneck of food supply in a nuclear war is perhaps 1 billion - the number of people that can be supported by subsidence agriculture without industrial support. The bottleneck of food supply after a decade of darkened skies and the freezing cold of endless clouds might be in the tens of millions.
There are some interesting sidelines; could humanity figure out something like turning lumber into food? Or perhaps humanity scrapes by with hydroponics. It could even be that humanity simply develops 10 gigaton nuclear weapons and simply knocks the Dinokiller around. (It’s not clear there’s a maximum size limit to H-Bombs).
Humanity has more resources and is more adaptable than dinosaurs. Remember too, that Dinosaurs did survive the Dinokiller–by learning to fly. A post Dinokiller Humanity is going to be different in ways that are hard to predict.