Cryptocurrency is best understood as a virtual commodity with modifiable variables that affect its value and utility.
Some examples include, transaction speed and cost, decentralization, inflation, and anonymity.
Cryptocurrency hasn't really lived up to any of these possible advances. In terms of a commodity, it's inferior to gold. In terms of a method of exchange, it's vastly inferior to a credit or debit card.
Commodities would have already destroyed Fiat currency if your average person cared about or understood inflation.
The reason why cryptocurrency will fail is that powerful and intelligent people have a vested interest in stopping it, and less intelligent and powerful people will fail to understand it.
We essentially already have cryptocurrency anyway in the form of credit cards. I can buy almost anything, in any currency, instantly with a credit card.
I find cryptocurrency evangelists indistinguishable from precious metal shills.
I hope I'm wrong, but I don't think I am.