> Cash is an anonymous payment system which cannot be traced ...

I just wanted to point out the possibility of tracking serial numbers on bills. Obviously it's not going to be as reliable, at least not now, but it could be made more so; for instance, if there was a sudden movement to use more cash for privacy. Of course, that seems unlikely; most people like the convenience of plastic/digital payments, and there's a lot of market pressure pushing people toward those payment methods, so deploying the infrastructure for more reliable cash tracking probably isn't worth a whole lot. But I haven't exactly run the numbers, and there are other possible incentives, so who knows..