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.
Decline is relative to what you envisioned, what you have, had and where you are going.
As I said in the OP, our decline is part of the arc of every single nation state that has existed.
And let me add as a general note, we are not special in that/this context either, it is what it is, in that this is not some phenomenon, some may argue no, we are not there yet, I just happen to disagree. The road signs are all around us if one looks at the past, the past of those nations states before us. The economy for example, is just a symptom, it always starts with the degradation of identity and society.
And, we are not the only ones, Europe has been beating us to the punch by a decade or 2. This isn't a woe is me thing, its the historical cycle of life *shrugs*