Well for me , before that would happen, bitcoin would have been legalised and replaced fiat or running very close with fiat. This would obviously mean more minning and then we could be thinking of that.
Bitcoin is not even illegal. Bitcoin just don't get recognition by the government or the government don't really talk about bitcoin because it will cause a hype. Some of the governments don't ban bitcoin because banning bitcoin will create a different effect. People are going to find out about it since there will be articles about it as well as announcements. Instead of burying the idea of bitcoin, it will serve as an advertisement for it. When they released a statement about bitcoin, people will be curious about bitcoin and why is the government banning it. And so they will learn that you can transact with people from the other side of the world by just having to a pay a couple of centavos. This is such a huge advantage for the people because sending money across countries costs a lot. With bitcoin, it costs almost nothing at all.
I do agree that before this happens, bitcoin would have already replaced fiat. I can't imagine what the world will look like if everyone's using bitcoin and the government can't collect tax at all.