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?
I think both of them will exists forever and I also think that cryptocurrencies will never replace fiat simply because it is our traditional money. In addition, We all know that both of them have different feature but we use them as currency. I think that cryptocurrencies will become one of our currency in the near future. However, it will never replace fiat. Lastly, I hope in the near future cryptocurrency will recognize as legal currency which can able to exchange in some valuable things.