I don't think we're in decline. We are still, by far, the wealthiest nation on earth. Our rate of growth has slowed, but we still consume about 25% of the world's energy and we have the strongest military on earth, by a factor of about 13.
Our ideas and culture dominates the globe, and American-designed products represent wealth and luxury. Wealthy people in China don't want Chinese-designed items, they want American products, such as the I-phone or a Michael Kors purse. When that changes, we might actually be in decline.
A lot of economists believe that China will overtake our economy by 2050.