The probability that the Internet and supporting infrastructure remains stable and available in a world in which government fiat, or even just USD, stop existing is virtually zero, rendering cryptocurrency worthless in every sense of the word.
Ad hoc local networking may still exist sporadically in a world in which the USD or fiat in general completely collapses, but that's woefully inadequate for supporting cryptocurrency usage.
It also completely defeats the point of decentralization and trust less transactions. If an isolated community is ad hoc networking, they are, by definition, centralized, and extremely likely to be a high trust community. It's much more likely they have a far better means of transacting with one another.