Cash, the folding type at least is a simple promissory note. There have been times that I have seen power cuts, belligerent pos machines, rejected cards, no signal, no battery, etc. resulting in cash only transactions. In a "cashless" society people will have to revert to no trade or good old pen and paper and IOU's, otherwise known as promissory notes. Oh, we're back to cash ...
The thing with cash is that it doesn't need an expensive device, an external network, electrickery or even a simple card, it is the ultimate and the simplest way to pay.