If the US dollar collapses or experiences hyperinflation, and/or US banks all collapse, pretty much everything is going to be fucked everywhere, most likely. Especially because whatever's driving that catastrophe is probably disrupting lots of other things, as well. (A major war, for example. The global Bitcoin/Ethereum/etc. networks might become disconnected and fragmented in those scenarios. Let alone infrastructure for fiat exchanges or paying for things with cryptocurrency.)

I think these events are very unlikely to begin with, yet a lot of cryptocurrency maximalists speak as though they're likely or inevitable in the mid-term or near-term. But if they do actually happen, I think digital currencies are going to be the least of almost everyone's concerns at those times.