I believe that crypto will be the world's currency. However i think stable coins are more suitable. There are many things to do with the security to make sure that the world's economic can be relayed on it.
The world’s economic will began to crumple if something as volatile as cryptos take over the official system and currency. Stable coins like USDT might be a wise choice but would still have to be highly centralized. Government would never risk placing all control in the hands of the people as this will result in frauds in tax payment and give rise to other illegal activities. So, decentralized crypto as a currency is basically out of the sight.
Cryptocurrencies are already used all over the world and constantly more and more countries begin to legalize cryptocurrencies and recognize them as official currencies, this is just a matter of time when the whole world will already fully begin to use cryptocurrencies with its full potential.
I would rather call them as crypto assets than crypto currencies. Cryptos can never replace the fiat currencies but might coexist with it as an asset similar to shares and gold. They will be legalised and regulated, but no government will allow cryptos to replace fiat and be the official currency of the country. Economy of the whole country can never rely on something so volatile.