If in the current time, all countries are switching to Cryptocurrency, the world economy may have a problem affecting it. But sooner or later, the world will have to move to Cryptocurrency
Many dozens of years must pass before all of the counties start to use the cryptos as their main currencies. Yes, the mankind has already made a great step into the cryptocurrency world but still, fiat plays the main role.
Such companies as visa or master card did not need several decades to implement their technologies. Blockchain and cryptocurrency are improving much faster. So the cryptocurrency will not need a lot of time for its implementation around the world, it will happen very quickly!