I have less information about abroad but i must say something that we have to reason collectively,
Out of
195 countries in the world i know too well that most of the population (country's) are solely depending on USD, EURO, POUNDS, YUAN for major trade. I mean before anyone could trade among these 195 you need any of those currency to be able to travel across, and if only 6 countries decides to breakout from USD how do you think it will affect the US dollar?
There will be no much effects
The significance of impact would indeed depend of the size and economic strengths of the countries deciding to move away from US dollar. For example if China, Saudi Arabia, Russia and India work collectively deciding to trade bilaterally using their national currencies, it can potentially affect the value of dollar as they represent top 20 economies with substantial trading volumes.