It's not the problem of cryptocurrencies. The problem is that people. If they have to use cryptocurrencies, they will learn how to use it. Remember when debit cards were created recently. Now all the people use them. Becasue they have to use it.
There is no need to learn how to use because technology offers new easy tools so we expect the number of crypto users will increase always faster than before. Infortunetly, the great majority of earth population still doesn't even have access to bank accounts or even any other financial service.
I do not ignore bitcoin problems. But I think cryptocurrencies have the potential to replace fiat money in future. However we need both of them now because of some problems.
Simply because we can't yet make a complete independance from the traditional system, so we think how the conbianation between them both may bring a solution.