I don't think all the countries in the world will survive on this especially those who are not that technology updated. There are still many people around the world who doesn't know what bitcoin or any cryptocurrency is right. So they will be the ones who will suffer if this will happen. Cryptocurrency needs time to be known around the world and once that happen for sure the society will easily adopt the when all countries convert into cryptocurrency.
The world can rely on crypto currencies as an additional or mainstream currency piled up with local currency. If we stay realistic, not everyone country in the world is using advanced forms of gadgets or technology. For their convenience and reliability, we need provide them both options i.e. digital or paper currency. Educated people can use crypto.