I believe that Bitcoin's fate is already destined. It's going to be the digital gold, a form of asset which will regularly be used for reserves. If you see, governments are slowly making Bitcoin part of their strategic reserves and every day, new companies are following this trend.
Believe me, no country will try to replace fiat with Bitcoin. Even El Salvador didn't do that. Bitcoin, as a currency is very unusable for the country and its population but it's also very dangerous because of its volatility. It usually goes up but it also goes down sometimes and this creates a very unstable environment for people. Imagine that you are about to pay for a heart surgery, which costs $10,000 and tomorrow Bitcoin's price fell by 10%, that's a huge drop for a citizen. The country and people needs a stable currency and an easy payment method.
I seem to recall governments using Gold to back their currencies a long time ago. If they didit before, they can do it again with Bitcoin. Of course, there's a slight chance BTC will replace Fiat in its entirety. Especially when Bitcoin is decentralized. Governments are always striving for control/power over the economy, so they will keep Fiat alive forever.
Since many governments are considering adding BTC to their reserves, the pioneer cryptocurrency will ultimately be used as a hedge against inflation (store of value). Backing Fiat with BTC would be ideal, but not necessary. With many players in the game, BTC will continue to grow in value over the long term. It feels good to have greater purchasing power "hodling" BTC. Doesn't it?
Many governments still hold substantial gold reserves. Btw I believe that there is no chance for Bitcoin to entirely replace fiat. Bitcoin payments are fast only through Lightning Network, it's slow when you use it traditionally. When lots of people make transactions, fees go to the moon and even in a very small nation, you should expect millions of transactions every day. Bitcoin won't be able to handle that. While Bitcoin is not the best currency for payments, it's the best to be used as a reserve, asset. Bitcoin's price is only going up and its probably a million times cheaper to securely store Bitcoins compared to gold reserves, which need very high physical security.