It's highly impossible for cryptocurrency to completely replace fiat currency, even if there's a war or global crises, cryptocurrency can't be the last resort to earning. It's either the trade by barter system is introduced again or a local alternative is brought to place.
Remember, to access cryptocurrency, internet, electricity and technology will basically be needed. And all these can't be accessible if there is global crisis.
It's still a case to case basis when that happens. With the sanctioned countries, they've got aide through cryptocurrency transfers from people outside their counties.
Although I agree that it's going to be back to the old times where people will trade again through barter. But, don't remove Bitcoin to be part of that system.
With the modern technology, even some crisis happens. I think that solutions to provide access to internet will be there.