I strongly agree with you that cryptocurrency cannot replace fiat currency not only because of the current rate of adoption globally but the disadvantages surrounding the blockchain sector. A good example of this is the power supply outrage, some part of the world lacks adequate power supply and there's no way one can make crypto payment without having power on their devices alongside this is network coverage which works with power supply. I believe if improvements can be done in this part it would be a good one for cryptocurrency to overcome fiat although the transaction speed is something to be noted as well, because a store owner won't want to wait for about 2~5 mins before payment is verified whereas fiat payment won't take upto this.