Paper currency that you see, actually is a very small percentage of the overall money supply in any given economy. Typically less than 3% of the money is actually in paper form, whilst the rest of it, exists in digital form.
We are already gravitating towards digital money because that is what the pundits wants us to do (paper is now somehow bad) and the government cannot be more happier (reasons are obvious – digital money can be tracked).
Your debit cards, credit cards, and mobile money apps, etc. are all digital. Slowly and surely, the world is being told (poisoned?) that cash is evil and that you must go digital (I wrote a small post on this: Cash Is Evil – Faisal Khan).
The future, however, currency just might not exist. In Gene Roddenberry’s future, there was no money in Star Trek.