I was thinking this as well.. However, the issue we have isn't the change of CC processors (MC to Visa), but the banks handling the transactions worldwide. Goldman Sachs is a globally known company, similar to Barclays which may have institutions outside the US. Banks like Amex, BOFA, and Chase aren't as well known outside the US, which would make integration for users outside the US harder; similar to how we in the US wouldn't know what to do with, say, ANZ Bank, MacQuarie bank, or Challenge Bank in Australia.
While Apple could try to do this on their own, not only would they come under heavier regulation (FDIC; they couldn't do NCUA as they are not a credit union).......
... Riddle me this, everyone. If Apple launched a company (subsidiary or of some sort) that is not run or controlled by shareholders (read: not-for-profit) and ran things themselves, they could form their own credit union, and have the profits earned each month/quarter distributed back to the members. All they would need would be to partner with a CC company and payment processor (TSYS, Chase, FIS, etc.) to process the CC transactions.
That could be an option, n'est-ce pas?
BL.