I think there are two points about American decline.
1) Relative decline as an international power.
The US is being challenged as the world's dominant superpower. The rise of China and re-emergence of Russia mean that the US is *only* the first among equals. This is unavoidable as is simply about demographics and economics. China has a huge population and as it closes the gap in per-capita development then it will become the world's biggest power. Other countries like Brazil and India are also emerging as global forces.
2) Living conditions.
Real wages have stagnated and declined in the US (as in Europe) since the 1970s. This is a result of globalisation and also neo-liberal economics. Competition was always going to have an effect and free-trade has meant it is harder to have higher wages in a first world country. This is partly choice though, with Reagonomics and NAFTA trade deals playing a role in exaggerating the impact.
