Its really impossible for paper money would be gone because its being replaced by another crypto currency which is really impossible though. As you mentioned government can produce anytime fiat money if the demand arises because of population inflation.. The future of paper money is still strong and no one could able to replace it anytime soon.
The biggest reason of fiat not being replaced by bitcoin is that paper money is produceable, any government can print any amount of paper anytime they want, and bitcoins has just a limited amount which would probably not be able to fill the gape between these two.
Actually we can also produce paper money of bitcoin, but the big question is would it be illegal or not? Of course it is illegal because the government only produce a legal tender to be use a currency of the people, and even it is unregulated, it is hard to attract the trust of the people.
Well i don't think that is possible though. Bitcoin will most likely stay as crypto and if ever it will be accepted i think it would be more like a credit card use. Considering the challenges bitcoin would be facing, it's likely that the future would still belong to fiat