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?
Actually, there is a big chance that cryptocurrencies especially bitcoin will replace fiat but I think it takes time to wait and happen. As you can cryptocurrencies is keep on developing so that there are tendecy that it would happen in the near future. However, we should know that it is hard to happen simply because we cannot replace our traditional money.