I think fiat will eventually be replaced by a return to a fixed exchange rate system with commodities like gold and silver. After enough time, I could maybe see bitcoins being used along gold and silver in the same manner too.
Are you in anyway insinuating that the old kind of cowry exchange system will be replicated in this 21st century with gold and silver serving as fiat?
Sincerely, I don't harbour any thought of Bitcoin replacing fiat because that's not EVER going to be possible. The reason being that there are countries battling with electricity and Internet connectivity. Again, there going to be illiterate folks who don't know how to access the Internet even in countries where that facility is available.
More importantly, I am a Bitcoin enthusiast. My curiosity and excitement drove me to google and invariably here at BTT. But I know the crypto industry can't replace fiat.