Crypto will not replace fait but fair will support the crypto in terms of fair. Already in some major companies and stores are accepting are one of their payments.
I agree with you, cryotocurrency can be very popular but it is very impossible to replace fiat, i am sure that even in the future all people in this world will prefer to used fiat because it is more easy to used and it is not complicated unlike on crypto.
I've heard from my teacher that in the future the fiat money will not be used again, extinct. And I am sure it will happen.
So maybe when later the fiat money is no longer there then crypto will replace his position.