Cash, currency or plain old money (whatever you want to call it) is just a convenient idea that everyone agrees to go along with.
It's evolved from bits of metal, to bits of paper, we've used plastic cards for nearly 70 years, and now money only needs to be numbers remembered somewhere on a machine like everyone's personal score in the game of life.
"Cash" will stick around as long enough people believe it's worth something. The tell tale signs that suggest coins and notes may disappear sooner than we all suspect? The growing number of places dotted around London that now have signs on the bar or counter saying "Sorry Cash Not Accepted. Contactless Payments only Please."
But there will always be ways to exchange value anonymously when it's needed or wanted. Whether that's pre-paid cards, crypto-currency, a return to exchanging precious commodities (gold is always a good investment
), or some other form of brokering service no one has thought of yet.