I don't think many bitcoin users want their currency to become main. As for sure, we can't even dream about it for next nearest 10 years.
for the time is certainly real money is still the main currency. but we can predict that in future most countries would have difficulty getting raw material for paper. so it could have been bitcoin become an alternative. bitcoin a digital currency is environmentally friendly because it does not depleting natural resources.
I am totally agree that bitcoin is a digital currencies are environmentally friendly,
But if the issue of raw materials for paper money, I think the Government will always
have a stock about that. And it's (Fiat money) will always be a main currency.