Look at what happening around. The first cryptocurrency bitcoin was invented to reduce the hassle of third party and have borderless transaction. Isn't it true? Now, look at what happening. People are more into investing it than using it as currency.
Most interesting is people are most interested in coin which give more ROI, they are not interested in the currency-ish of a currency.
For example, some people are talking a lot about BSV, but if they have some research, they will never take it as currency, but as this is giving a good return now, it's best for them. Why man? Why don't we look into a decentralized coin? Why do we only take it as investment? Did Satoshi ever take it as investment? Or he invented Bitcoin for borderless payment and solution to the intermediaries problem?
Indeed, replacing of cryptocurrency to fiat currency would not happen just like what other says about it. It will be really hard for us to use cryptocurrency as a standard currency in real world especially with this feature of being volatile. How can you imagine to use bitcoin or any cryptocurrency to buy some foods on market or buying some foods with lower price. The inventory system would be hard also for them to record because of changes of price time by time.
Cryptocurrency will be only as an alternative currency for fiat currency that is how cryptocurrency works.