Theoretically speaking, we can think of alternative realities having btc as the main currency. However, in reality, I don't think it will happen in our lifetime. Each country will stick to their main currency and the possibility that I am thinking is just, they will develop a digital currency corresponding to their main currency. But making btc as the major one, I don't think so.
At first yeah, it may seem it is but then if we think about it, there is now BTC adoption and I think there is that island called BTC-island. So maybe time will come that this obsession can go severe that they will even made BTC as their main currency. The one that you are thinking there about digital currency which corresponds to the main currency of a country have happened already. Have you heard about CBDC?
We also have stable coins and I think both have a lot of similarities. BTC even as an alternative currency, is already playing a major role on one or the country's economy but how much more if they fully embraced it? The impact can only elevate.