Bitcoin, the first and still the most popular cryptocurrency, began life as a techno-anarchist project to create an online version of cash, a way for people to transact without the possibility of interference from malicious governments or banks. A decade on, it is barely used for its intended purpose. Users must wrestle with complicated software and give up all the consumer protections they are used to. Few vendors accept it. Security is poor. Other cryptocurrencies are used even less.
Economists define a currency as something that can be at once a medium of exchange, a store of value and a unit of account. Lack of adoption and loads of volatility mean that cryptocurrencies satisfy none of those criteria.
But as things stand there is little reason to think that cryptocurrencies will remain more than an overcomplicated, untrustworthy casino.