The US is in no position to decline:
1. Most of the international investment coming from Asia (China, India, oil-rich Middle East, Pacific) and South America goes into the US. The US has become like an investment bank holding trillions of other people's money. Noone will let you default on this :)
2. As inefficient as the US government may be, when it comes to the (private) business side, the US represent the most efficient (and inhumane from a social perspective) environment. For us, Europeans, you're a bunch of workaholics, obsessed with competitiveness and professional success, so we're happy to let you do all the work and push the world forward economically and scientifically, as long as you don't get so crazy as to invade us all :)
Sure, many people would be happy if the US falls, but surely not the ones with financial and political power. You need to keep working for the rest of us all to be able to have a life :)