Personally, I will never see this happen. Crypto will exist but fiat will exist as well. They will co-exist because they have different features.
We both need fiat and crypto. In my opinion, we can reap the benefits of both crypto and fiat so why any of the two should disappear?
in my opinion, everything can happen and everything is possible if the development of technology and the development of the era requires currencies that are the same as cryptocurrencies, surely fiat currencies will be replaced