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.
Cash will eventually get killed by digital money, not necessarily bitcoin but all the governments across the world which have a good technology development starting to switch to digital money than using a rotten fiat "paper" money.
So basically in our life times, it could happen, technology developing so fast as you know, maybe a decade ahead, everything will be cashless, and we'll only need to use digital thing, it's good for environment and economical swell.