I have a vivid discussion with my friends lately. They say that one day people will forget what cash is and they will start use card only. Then the era of the changed and innovated cryptocurrency world comes, and we will stop using money and switch on crypto coins only. They say it might take twenty or thirty years, maybe longer, but it is a sure thing to happen, they have no doubts about it. I am not so sure... well, people might give up using cash as it is not that convenient, but giving up money as whole? I really really doubt it, it seems so impossible for me. What do you think? Maybe we will start using something brand new, neither cash nor cards? What is the future of the cryptocurrency world, anyway? Will it die in fifty, sixty years or it is going to be a thing that will live forever? Let's dream for a while 
I think there are already discussions and countries are discouraging the use of cash. Online transactions and exchange is the new way and its successful.
So yeah, if its just about cash, we will see this in near future.
With respect to crypto, I think replacing the government approved money is not going to be as easy as thought. There are various agents that will try their best not to let this happen, since crypto takes away their control and power. But crypto will make its way and we must keep our faith high.