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.
I've never been under the illusion that Bitcoin will become a global currency. The government will never allow that. Bitcoin becoming a global currency will mean that the value of the local currency will depreciate. Suppose people now try to convert all their money into BTC because of the devaluation of the currency, the local currency will depreciate even more, and the government doesn't want that.
All I want for Bitcoin is for people to be allowed to use it whenever and wherever they want. It doesn't have to be legal tender or approved by the government, as long as it's not a crime to use it and the government doesn't launch campaigns against it.