Another important reason by the US is likely to continue to flourish for a long time: nukes. If you look at the history of warfare, technological superiority, be it tactics, equipment, etc. nearly always meant dominance for those wielding the technology. If there was ever a serious threat to the US, you can be sure that said threat would be vapourized. In the shorter term, the US (and other nations) needs to find ways to drastically reduce the enormous reliance on natural resources (assuming that these resources are finite).