in my opinion to replace the fiat currency is not possible because the legality of the crypto currency is not recognized by all countries in this world and they still believe in the fiat currency of course
We do not know anything about our future. Probably, the writers of science fiction were right, and robots will replace some people in the future. If it happens, the cryptocurrency can also replace fiat.
Your thinking does not make any sense to me because at some place you are not agree to consider the value of bitcoin and on the other hand you are comparing robots to human. So this is totally double statement. According to me crypto currency is our future and it will not take much time to become more valuable than fiat.