There's no such thing as forever everything when served it's purpose will die or will be replaced by a better one, but Cryptocurrency will stay because we need it in our present generation, it is good for the next 50 years if you ask me.
I don't know whether it will survive for that long. Technology is getting better with every passing day and eventually the supercomputers will be able to crack the private keys in the current format. I don't know how much time it is going to take, but my guess is another two decades. And once that happens, some better technology will replace the current concept of cryptocurrency.