I think that cryptocurrencies are not to replace fiat money. It's not their main goal. They are a great alternative way to use and invest into. And as they are volatile, it's really difficult to see them being the world currency or the main currency of a country. And I'm pretty sure that they will stay like this in the future also.
Yes, indeed Bitcoin and cryptocurrencies will not replace fiat. However, Bitcoin and cryptocurrencies will not be like this forever, because every time Bitcoin and cryptocurrencies experience development which is a progressive rate of a powerful but not well-known technology.
I support Bitcoin to be more powerful, and cryptocurrencies are starting to become a mainstream payment tool, at least as an alternative payment gateway for those who want their privacy to be maintained.