This is more of a fantasy kind of question. Unless there was some sort of catastrophe that ended society as we know it and electricity, you will need for every person in the world to cooperate in not using or create new cryptos.
This is not a fantasy, if government will ban cryptocurrency and there will be a hard implementation with it, it will happen and the society wont take a risk if there will be a punishment using it maybe it will drop users rate from 90 to 95%.
If the government will really ban crypto then for sure that it will be too risky for all of us because it's hard to disobey the government as of course we don't want to suffer from the consequence. If that happened people will really have a hard time in transacting crypto currency and that will reduce the demand for it.