It isn't as simple as "divert the trucks carrying corn and soy from feedlots to grocery stores".

Humans can't just eat uncooked, unprocessed corn and soy. Most people don't know how to use them as ingredients. You'd have to retool or rebuild all the food processing plants that current deal with animal products. Ramp up oat/soy/almond milk production. And so on.

> unprocessed corn and soy

You mean edamame and sweet corn? You can grab an ear of the latter right off the field, husk it and eat it raw if you'd like. Or is there something about megafarms that prevent that?

> Most people don't know how to use them as ingredients.

If they get hungry they'll do what people have done since time immemorial and boil them. No-one will starve sitting on a cache of beans. For that matter, humans can eat pig feed directly (it's not as safe as human food, but you can boil it too).