Nice article! Yes, cryptocurrencies will obsolete fiat much the same way the automobile obsoleted the horse and buggy. It's simply better money.
I am for cryptocurrencies in general and a year ago I thought that this is where the world is moving, this is how things will get done in the future. I think we all remember discussions about times when one could buy a cup of coffee or some goods in the supermarket and pay via bitcoin. Now nobody says such things because it is obviously impossible. Perhaps the currency which has chances to become new fiat is dogecoin, because it is cheap, safe and fast. But even a good currency can't become a part of everyday life yet. As the main point of currencies is decentralization, owners of online wallets, trade websites and markets are unknown. This leads the to temptation to steal of the money and disappear. Sometimes they get caught but mostly they aren't. I think that cryptos can replace fiat only by losing anonymity at least in important cases like with website owners. And they have to be regulated by laws so that owners of markets where people lost money really take full responsibility for that.
Sure in a short term cryptocurrencies won't completely replace fiat money, but it is higly possible in the future. For today banks, even government structures actively try to use blockchain technology. Some of them attempt to create their own cryptocurrencies or estalish some kind of control on existing ones. I think they'll figure out the way to take cryptocurrency under regulation and control.