The problem is the safety of our account, i believe when crypto replace fiat it means the value will be more stable and the transaction could be fast as fast using debit card. But the security of our account is so vunerable, or maybe there will be a wallet that cant be hacked.
For large transactions, cryptocurrencies can be used, but can you imagine when a 7-year-old boy buys candy to pay using crypto?
What happens is clearly a payment problem. To replace fiat money, I think it's impossible. But to align with fiat money is very possible.
Only functions and uses can differ depending on needs. Crypto money is a sophisticated technology that can definitely add value to financial transactions because digital currencies do not have physical, but banknotes cannot be replaced because they are practical and have physical properties.