>The other 80% of the bean would have to be discarded if not used for meal to feed chickens and pigs
No. That is how it is currently used, but in an emergency you could make soy flour, tofu, etc. The calories are consumable even if it's not what we typically eat.
Or, more relevantly, the land that is used to grow soy could instead be planted with corn (or perhaps winter wheat in a post-volcanic scenario). Right now we're planting out crops to optimize for market value, which is driven by livestock feed use. If we instead optimized for human nutritional value, we could easily double output by mostly eliminating livestock (even accounting for the loss of nutrition provided by the livestock).
>Needless to say though, if our agricultural systems are producing 73% less crops and livestock there will be ~73% less humans.
An incredible oversimplification.