It is really interesting to see how much influence Bitcoin will have on this. Altho days of USD as world reserve currency are already counted down, Bitcoin can decrease them drastically. It can all happen just in a decade.
I would wait to see at least 1 country officially accept Bitcoin. That would mean a real new threat to USD.
You need to understand that individual countries do not "accept" any currency, except for their own tax payments (they only accept their official currency for this). Countries will have their own national currency (or will adopt the currency of their economic zone, as in the EU, or of another country, as is often the case with the Dollar) however the country does not mandate that all trade be done in that currency. People of any country are freely able to engage in barter transactions in which they exchange some other form of payment in exchange for goods/services
