Paper money has so far been recognized as the most optimal way of financial settlements between individuals and legal entities over the past centuries. Despite the fact that in recent years the world has largely switched to the use of non-cash forms of cash payments, cash, in my opinion, will continue to exist for a very long time, and maybe always. It will not be affected especially by the introduction of crypto-currencies into our lives. She will walk only in parallel with cash.
I'm tremendously curious who and how exactly established that paper money is "the most optimal way of financial settlements between individuals and legal entities over the past centuries". Paper money itself has been around only for a couple of centuries, so what do you mean by the past centuries, how many of them precisely? And I wouldn't call it the optimal way. Personally, I'm quite happy with cashless transactions without discriminating between individuals and "legal entities".