I think it is impossible to replace with conventional money, even the government alone is not possible to allow it.
Our governments can never allow this to happen that is why in some countries bitcoin has not be legalized because the see bitcoin
as a threat which can replace conventional money and lead to the collapse of the systems used to regulate cash flows.
Their only bet would be quash any sort of national interest in Bitcoin before it gets anywhere, otherwise, they might end up destroying their economy if they try to get rid of Bitcoin when it is already established as part of the system. If they take too long to do something like that, the game is over and Bitcoin would essentially win, and the government would have to design itself around that.
I assume this would happen at around 10%-25% participation.