Adoption rates don't imply usefulness. The only reason cryptocurrencies have become popular to mine or invest in is because people are hoping to get rich quick with minimal effort on their part. As far as an actual currency system goes, their value tends to be hugely unstable and entirely speculative in nature, transactions are far less efficient compared to other digital transactions, they lack protections against fraud, and hardly any businesses accept them as a form of payment. And as they gain more public attention, governments are increasingly looking into ways to ban or otherwise regulate them. Some cryptocurrencies will probably stick around in some form, but those expecting the recent gold rush to last are probably expecting a bit much.