It can not happen that crypto can kill fiat money because the whole world still uses cash in different transactions. It can not wipe out fiat currency, because btc is accepted only at some selected business establishments and, the crypto is prohibited in other countries.
I agree that cryptocurrency wont kill or replace cash. Cash is very convenient for smaller transaction like buying basic commodities or fare of public transportation. It could be cryptocurrency will be added as payment method just like credit card not all people has credit or debit card. So cryptocurrency is very helpful for online or bigger transactions.