It will be impossible to replace completely cash with crypto, even in case of general acceptance.
More likely, you'll have the two systems working in the same time.
Then, who know, somewhere in the futur, when everyone will have a mobile (or a chip under the skin...) then there won't be need of cash anymore.
That's going to take at least a hundred years or so, so I wouldn't worry about.
Unfortunately not gonna happen in our lifie times i think.