For me, it is a certainty that cryptos will never replace fiat currencies. There will be a parallel economy and the btc will become a currency like USD, EUR etc... with a parity for each pair, but not a substitute currency. And that's good, because the global economy would be shaken up, it wouldn't make any sense.
Yes cryptos have it's own use and it can never replace the real fiat currencies and even if we start using crypto instead of real fiat then it will create a chaos which might lead to a disastrous economical recession. Cryptos can be used as a mode of money transfer and other new technologies which gives i upper hand compared with traditional method like money transfer system which takes m\weeks if we do it through any money exchange but it's a matter of seconds through crypto.