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?
True both are very usefull and fiat will not be replaced by any crypto money instead they will both help other to mentain each usage,,,and i beleive that really no one will be gone as youve said mate it will stay forever qnd be our forever partner.