Thanks for your great question!
I'm not a futurist, so I'm not the best qualified person to answer this question. However, I'll give it a shot. I'd like to hear what other people think about the matter - you can contact me by using the feedback form. To the question: I don't think paper money will completely disappear, but electronic transactions have become more and more common over the last few decades - I see no reason why this trend will not continue. We may get to the point where paper money transactions become incredibly rare. At that point paper money may act as the backing to our electronic currency, the way the gold standard once backed paper money. That would be kind of unusual, given that gold has an intrinsic value-in-use, whereas money is just pieces of paper.
I'm not sure if we'll ever get down to one single currency, though I suspect the number of currencies will fall as time goes on and the world becomes more globalized. We see that happening today - when a Canadian oil firm negotiates a contract with a Saudi Arabian company, typically the deal is negotiated in American dollars or Euros, not Canadian dollars. I could see the world getting to the point where there are only 4 or 5 different currencies in use. At that point we'd likely have a battle over standards, such as we had with VHS vs. Beta VCRs, Apple vs. IBM-compatible computers, and Blu-Ray vs. HD-DVD media players.
If that happens, then it is likely that soup could be purchased in the same currency in both Malawi and New Delhi. The prices could vary between the two locations as strict purchasing power parity is unlikely to hold for a product like soup, for the reasons given in A Beginner's Guide to Purchasing Power Parity Theory. Thank you again for your interesting question. I've been thinking about it quite a bit lately and will likely continue to do so for some time.
