Cryptos haven't replaced the fiat yet and it won't in future too as many economies have declared it as an illegal tender. Without complete support and consistent flow cryto cant replace the fiat . Hope you got me! 
There is nothing in this world which you cannot expect to happen and that you never know what will happen after sometime. However, I am very much sure that these crypto currencies can be much more beneficial for the economy and that they can help us in much better way than that of the fiat. The day government will support these crypto coins will be the day we will see crypto currencies taking over the regular currencies.