What I haven't even considered is that although there's a few cases where your immediate community can work together on stuff like this, an actual collapse leads to a power vacuum and breakdown of supply chains, so your immediate community needs to not only be self-sufficient for food but also defence from looting, and yet somehow also not important enough for to the original government to try to keep running, nor for any warlords who spring up in the vacuum to try to pillage, and also not for any foreign powers to peace-keep or annex.

Hmm, I wonder: if the USA suddenly collapsed as hard as the USSR, would Mexico and the Texas region start fighting? Or Cuba and the Florida region?