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.