ApplePay is a substitute for physical credit cards, not cash.
Those damn chip cards take almost 10 seconds to do whatever validation they require. Used to be: whip out card, swipe, sign and go. Now I find myself standing there waiting the additional time for the damn card to be approved.
ApplePay w iWatch is now faster than swiping a card. It only takes a second or two to validate and you are done, with the added bonus of not needing the dig out your wallet at all. It's faster and easier. It's convenient.
I wouldn't go out and buy a Watch for this purpose, but it's a slick value add.
Cash is not about convenience. You have to get it, carry it, make change with it. Cash is a different tool.