Not replacing your credit cards... you're WALLET
Apple realizes (I think) that the goal is not replacing a credit card. That's a short-sighted goal. Apple WILL replace the credit card, of course, but as a stop-gap measure. Swiping your iPhone will charge to your iTunes account, which is tied to whatever credit card you want. Eventually this middle-man route also leaves Apple to negotiate better deals or remove the credit card companies altogether and you'll not notice (or care) because the system will be the same; you'll still be charging to your AppleID. But, like I said, this is all stop-gap.
The real goal is to replace the entire wallet and maybe even my keys. Why replace the credit card when I still have to carry around my driver's license, passport, and other IDs? I'd still have to carry a wallet anyway.This is a longer term goal, but significantly better. Your phone trusting that you are you is the first step.