It will take more than dozens of years until we can see a change on this kind of economic system, yes, it would more than impossible.
I think maybe but not now, it is possible because there are other people doesn't know about this technological process of using digital money or currencies in networks. Maybe cash will extinct if all things that we used are made through technologies.
Yes, cryptocurrency cannot replace cash requirement, but if a government of any specific country complete digitization upon its financial system, I guess it will be possible. But there will be roadblocks while doing this, but cryptocurrency can helo them. Centralised crypto can be implemented and a system can be built which will complete the payment with a fraction of seconds. But yes internet will be the biggest problem, I doubt there will be 100% uptime always anywhere in the globe. That's kind of tough to happen.