Cash will become obsolete as the global economy pushes further into every individuals life. It is inevitable. A true global economy can not function on paper money. It is simply too slow of a process to transfer, print and destroy paper money, and coins cost more to make than their worth in some cases, causing the people billions of dollars per year. (Looking at you Mr. Penny).
I can go without a wallet every day of the week in California. NFC is accepted in lots of places I go, and while I still carry one credit card and some cash, just in case, I rarely need it. And I do believe that if Verizon and AT&T weren't fracking d#cks about Google Wallet, NFC would be more widespread already. Instead, we have a system supported by the carriers called ISIS, that is over budget, behind schedule and doesn't work.
If Apple does add NFC, it will grow in the US. Assuming the carriers allow it.