Below is an interesting chart reflecting the life of various currencies since 1400. The current wisdom seems to be that the dollar is doomed. I think in reality this is true, BUT, the majority of world trade is still carried out in USD, even though it is changing rapidly. The financial systems are mostly USD orientated.
The US has the largest and most diverse economy in the world, although the strength of the Chinese economy appears to rival the US economy in many respects. The Renminbi is however not traded worldwide yet, but does have an established international bond market which is growing and presents real competition to the USD.