If we think of other variables than what Vitalik said, there are less upside to crypto completely replacing FIAT.

Singular govt and singular currency for entire world can be a threat for many religions and cultures. I mean we are already being under attacks from certain religions every now and then. And on the top of that if singular currency happens then attacks would be lot regular and brutal among the borders. World is definitely not going to be united under one govt and it's not a good sight either especially when we all know - china has the power to put down both US and Russia and they trying to sit on throne of the world is bad for the world. 

My 2 cents.