How does capacity work? If the US needs food because of an export stop of supplier countries because of a crisis, those farmers can't swap out what they produce. Depending on the year of time it might take 12 months to swap out export products for something else. This is not "standby-capacity".

Many things that can stop imports take a lot longer than 12 months to resolve. Wars can take several years for example.

Getting a domestic replacement in 12 months of belt tightening would still be a lot better than having to deal with several more years of that.

Also I'd expect that there are cases were we export X and import Y, and if Y was cut off we could switch to using X somewhat as a substitute. I don't know offhand of any specific plant foods where this is the case, but pork and beef illustrate the idea. We import about 15% of our beef. If that got cut off we could cut some pork exports and use that as a substitute. It's not a great substitute but it is calories that can get you through a crisis.