Or, everyone rallies in the first few years and the few survivors that are around work together and share resources, at least long enough that everyone gets theirs.

Yeah, the problem is: we're both right. Some will go one way, some another - and when the two types intermingle, the results would appear to be inevitably regrettable.

Movies and video games are more entertaining when there’s over-the-top conflict, and I think that shapes our predictions.

Having lived in big cities where "bad neighborhoods" means one house out of a thousand might harbor some bad kids who go out and do bad things, I'll say: the vast majority of people in most of society are basically good, some of them truly great, but it doesn't take very many bad actors to bring down a whole 100,000 population area into fear and chaos and over-the-top responses to threats.

You want examples of over-the-top responses in real life? They're rare, but cops doing outrageously terrible things isn't just in fiction... a lot of real life is more cold, callous and brutal than a lot of fiction.

we’d learn to use horses/cows to pull the tractor implements before the tractors are kaput for good.

In a lot of ways, horses and cows are much more work and expense (inputs of valuable materials) to keep operating than a tractor, it's why the horse drawn carriages died out so quickly - not because they're slow or smelly, but because cranking up an engine that sits in a garage and waits patiently for you for days, or months, at near zero storage cost is infinitely more efficient than protecting your livestock from disease and weather and poachers and wild predators...

And, yes, in those places where people can maintain working animals, they will go to that expense to plow the earth by mule or ox or horse power instead of planting by human labor, but it is a big step up from desperation on the run to keeping beasts of burden in working order.

The biggest danger would be illness and injury.

That is a big one, particularly while the cities are filled with unburied rotting corpses.