I think if you're not aiming to be fully self-sustaining it makes a lot of sense to buy things like these and focus on growing fresh fruits and vegetables. From a sustainability perspective beans/wheat/corn and to some extent potatoes are fairly easy to store, making them suitable for central/slow processing. I don't have numbers on this but my sense is that a lot of the resource usage of our agricultural system is due to perishables - food that needs to be cooled, that gets thrown away if no-one buys it on time, that's easy to damage in transportation, and that has high water content (making it heavy to transport).