Well if the cryptocurrency gets really popular in the future it might happen because in the future everything we buy will probably be online.
So i think that it is possible for cryptocurrency like bitcoin can be such a important coin in the future that it will replace paper money.
I agree but even bitcoin is very popular in the whole world, it is not enough to replace fiat. Fiat will be replaced if it getting worst because of inflation that make it worthless, people will try to solve this problem, most probability is to find the alternative such as bitcoin. So there still probability for bitcoin to replace real money.