Cryptocurrency was a good idea, and some better implementation still could become something really useful someday. While there are plenty of lawful uses, for now, their primary appeal seems to be for pump and dump schemes, gambling ("speculative investment"), ransom payments, tax evasion, circumvention of financial rules, and other unlawful uses.
It's just a matter of time until regulation, taxation, and law enforcement catches up and squashes most of them.
Not to mention that they are resource hogs (hardware and energy) and goes counter to where the world needs to be heading.
I think for a successful system, it needs to be open source, issued/backed by a neutral central global master authority (eg. UN/IMF/World Bank) that can issue new coins, but still be somewhat decentralized, and be cost and energy efficient.