Besides, what will the world use for a reserve currency????
There is no other currency that is large enough to facilitate the international oil trade let alone all other trade. The Euro, the only other large currency, is making some inroads but is far from taking the place of the $US, especially lately. There is not enough Yen or pounds or any other currency to take the place of the $US and the EU is just as indebted as the US with about the same size economy. Besides, the US still has a lot that the world wants, everything from soy beans to John Deere tractors and if the $US ever did fall from dominance its value would plummet making US exports even more competitive in world markets.
China is currently building a highway network bigger than the US interstate system and a high speed rail network larger than the entire EU system. It is also building over 100 entirely new cities. This uses a lot of domestic savings. Once these projects are substantially complete China will begin to make this money available for consumer finance. Until then, it is not considered possible. China needs trade with the US more than the US needs trade with China. While China has a huge trade surplus with the US this basically covers the trade deficit China runs with the rest of the world. A big problem for China in moving to a consumer economy is how it can continue to pay for its imports of raw materials and food as manufacturers move from export to domestic consumer market.
Besides, US fiscal problems are due to simple stupidity and the US government debt is just not as big of a catastrophe as you seem to think. The only problem with deficit reduction in the US is that all reasonable solutions have become hostage to venal politicians and their idiotic misrepresentations of how economies actually function. If this situation continues for another decade or so, which is likely due to Republican intransigence on taxes and the wilful ignorance of US voters, the US and its people will be deserving of even your most unlikely scenario. But, as far as any of your predictions happening before the end of this decade, not very likely.