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%.
how do you see that come to life? governments can't agree on lesser things
I don't believe there could be something global that the majority of the countries will agree upon
bitcoin can only be banned if internet is banned , even if it was declared illegal , there will be people using it
just take a look at anything that is illegal , drugs are illegal almost everywhere , do they vanish , lol ?
it is a fantasy and a bad one , I would not want that to happen
but again , if it happens , then there will be no tears in my eyes - it would not be the end of the world for me