If there is a not total civ level collapse which still fractures the internet into different networks which do not communicate, I think crypto will be even more fun, as then you can double spend without needing 51% if you have trade the same coin at the same time on both networks. Not trading anonymously/trusting that you can find the guy who you sold a thing too later to reverse the transaction, would fix that of course but wasn't that one of the appeals of crypto?

Say if I was a Russian olicharch with a lot of crypto and Putin decides to close the digital iron curtain, I would dump all my crypto at both markets at the same time the moment the curtain is down. Sure eventually your crypto gets owned by either a russian or a non-russian due to the consensus algo but that is their problem not yours.