Hybrid seeds aren't used for all crops, not for wheat as I recall, I'm not sure about soybeans. Corn is of course the big one, but at least going forward humans wouldn't particularity want to eat corn if wheat is available.
But I agree with your bottom line, on the problem of big cities, well, Dean Ing, in the appropriately titled novel Systemic Shock, put it this way:
The American public had by turns ignored and ridiculed its cassandras: city planners, ecologists, demographers, socialists, immigrants, who had all warned against our increasing tendency to crowd into our cities. Social stress, failure of essential services, and warfare were only a few of the spectres we had granted only a passing glance. We had always found some solution to our problems, though: often at the last moment. Firmly anchored in most Americans was the tacit certainty that, even to the problem of nuclear war against population centers, there must be a uniquely American solution; we would find it.
The solution was sudden death. A hundred million Americans found it.
It's the water supply that would be the big killer, you can't go without it for more than a few days, and that just wouldn't be enough time to get enough to everyone.
I'm not sure they'd be so successful in "pillaging and destroying any remaining food pipeline" ... how do you to that to an interstate highway? At most, to the fueling stations, but those are a fairly small scale, and I would expect easily replaced thing. But that would also help to self-limited the radius of the pillaging and destruction, keep it mostly away from the farms themselves, which aren't exactly defenseless if such is needed for only a relatively short period of time.