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.