The world often behaves in strange and mysterious ways. Even though the global economy is collapsing, central banks aren't showing signs of fragility. They will remain in power as long as governments support them. And people can't do anything about it.
Bitcoin was once thought to be Fiat's replacement. But so far, it has failed. Especially when many consider it to be a store of value than a currency. Using BTC on top of Fiat is only a pipe dream. At least, we have a choice. As long as BTC remains decentralized, there should be nothing to worry about. Who knows what measures central banks will take to protect the economy in the future?
Was it? I did a quick check and couldn't find the word "replace" in the Nakamoto white paper. I bring this up because I think this is one of these catchphrases that some people (or many) quickly fell for and propagated it themselves. If you asked some people what they know about Bitcoin, this is probably one of the most provided answers while they couldn't explain the technology in any further detail let alone provide the core principles that Bitcoin is based on.
If anything, Bitcoin drove the price up for the legacy system to remain shitty. Remember how long it took for fiat transactions to be settled between banks? No need to discuss intercontinental transfers... Only after Bitcoin emerged, they began introducing 24h guaranteed settlements, then upgraded to instant settlements within the same currency unions.
Now they are embracing Bitcoin because the risk is high to not do it. One bank did the first step, others followed. First they didn't want to be the fool who has been the first, now they don't want to be the fool who missed the boat completely.
A lot of things are happening and this idea that Bitcoin will replace all cash in the world is nonsense to begin with. Bitcoin can become a decentralized geopolitical financial tool in a very complex world and countries might be competing for domination of this geopolitical financial tool. But they also want a tool where they can plan or with some certainty anticipate the effects of their financial political actions. Giving up national fiat currency means that the player either probably has sufficient power over Bitcoin, but that would imply other countries wouldn't be well advised to give up theirs.
It is hard to imagine that Bitcoin will be so uncontestedly powerful as a financial system that countries start rushing one after another into it, abandoning their currencies and make Bitcoin the one and only currency.
But it's an interesting idea to think about and it is not like I would say no per se because I don't know. But saying that "Bitcoin was once thought to be Fiat's replacement" is something that some people might have thought without understanding Bitcoin, and I doubt that it was the opinion of the majority of Bitcoiners, and I doubt that Satoshi thought he is about to kill Fiat once and forever. But he saw the need for an alternative system that can exist outside of the Fiat realm.