Cryptocurrency is just a fiat in a modern world. They can be distinguished in some nature but their uses and relevance are comparable. The good thing with crypto is that it is decentralised and cannot be controlled by one. It also has its fixed nature.
The "only difference" you talk about is a complete change to the current financial and monetary market, so it's not just a difference. In my opinion, centralized FIATs will not disappear for decades or even hundreds of years. I think cryptocurrencies will just take a piece of the financial cake and will exist alongside other forms of payment. Everyone will be able to decide which one they want to use.
Of course fiat will not disappear from circulation, considering that there are still governments that seem to choose to use fiat instead of cryptocurrency, this can be seen in my country, where until now the government seems to have clearly stated that it has not been able to legalize cryptocurrency
It's not just national governments that will make problems for the adoption of cryptocurrencies. I think that despite everything, the difference in technical progress in many countries will not allow to adopt cryptocurrencies as a form of payment for a long time. Paper (or plastic) is much more available in many parts of the world than the internet, so that will definitely be a big barrier as well.