But many if not most cereal grain farms (plus legumes besides soybeans?), the ones that count for getting the most calories to the most people, are quite some distance from big cities. And if you only have to deal with an initial wave, you can do a Maximum Effort to keep them out of your fields, it's long term banditry where farmers by themselves inevitably lose because they can't do both at the same time.

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.