Why would fiat disappear, and more importantly why would an objectively inferior currency like bitcoin replace it?
 Bitcoin is practically unusable as a currency because of it's absolute failure to hold a steady value.  People rail on the USD for depreciating slowly but predictably, but think that something you won't know the value of tomorrow is somehow a better system?  It's completely idiotic.  If bitcoin was a viable currency, the concept of stablecoins would never have arisen.  Stablecoins arose to do the one thing bitcoin will never be able to do- hold a predictable value, which is the most important aspect of a currency.