No Us dollar is national currency of USA,any country need to have his own currency,look euro currency ad look what is happening with Greece and Portugal
yes, that's right, it would be very difficult for bitcoin transaction to replace the tools typical of a major state dollars. I think it will not happen, and may bitcoin will just be an alternative, but the function is more like a credit card, but most likely bitcoin still has a more practical use of the credit card
It would even be impossible for usd to be replaced by bitcoin now, and in the future. The reason is that with usd, the big dogs are able to control usd and not bitcoin that is why they will use all their power to stop bitcoin from overtaking usd or any local/int'l currency and it's clear that in some country, it's already happening.