Your argument is invalid. WoW has gold as a limitless farmable currency and wow economies don't collapse. You have not provided a conclusive argument that diablo 3 gold will become worthless.
Yes there will always be some kind of high level item or rune that might become a currency but in d2 you could offer a buttload of gold for a shako and get laughed at. In d3 I expect to be able to offer gold and not be laughed at (it would have to be a lot of gold however).