Crypto will not replace Fiat, there is just not enough exposure. Yes crypto is more secure but the average person will not use it on a day to day life, at least in my eyes
Or in other words, the demand for fiat is way too high and it is not going to be affected by the expansion of crypto through out the world because not all the people throughout the world have access to the technology so crypto will stay for long. In addition, a fraction of population prefers to keep cash as well because at times, when the internet is not accessible, you might need it for emergency. The problem of liquidity also matters.