Well, everything is possible, because look at how technology grows these days. It is possible for the world to have one global currency, but it's not appropiate for now. Because too much changing could cause a disaster. But maybe one day, it could happen.
I do not think so because countries are in competition with each other and they will never leave their currencies which are at the center of todays business. On the other hand, if crypto coins especially the Bitcoin even if gains the highest strength it has nothing to do with other currencies. Bitcoin and other crypto coins will exist in future but it will have effect on fiat and it is completely wrong that fiat will disappear.