Will bitcoin ever replace fiat currency? question is like will the internet ever replace the fax machine or not?.
Even with my view that bitcoin might not totally replace fiat, I think that your analogy with fax machine and internet on one side and fiat and bitcoin on the other side does not correlate. Fax is what is consider now as old fashion because internet has totally replaced it, in fact not even internet but computer has taken the roll of the said fax functions.
As for fiat, it is used currently while bitcoin is yet to be accepted, bitcoin as it were, plays the digital roll for just few people. So the comparison isn't adequate.
Again, we can also factor in the level of illiterate people that will continue to exist in the world no matter what education advocacy that is adopted. Using the internet (which Bitcoin and by extension cryptocurrency is tied to) comes with a certain level of literacy (I am not talking about classroom here). Fiat was swiftly able to replace cowries and trade by barter because both operated almost on the same level -physical exchange. Cryptocurrency is different. And to that it can't replace fiat. Both will exist side by side.