The world economy is huge and growing, and the US economy is damn strong with a significant share of it. It also owns the world as far as raw military power and power projection goes. The US would absolutely use its huge military and economic advantages to keep its position as top dog if necessary. It is fine that the world's economy is growing (inevitable after the devastation of ww2, which barely touched the US; also industrialization in countries like china), but it doesn't mean the US is any weaker for it. And anyone who thinks the US won't keep its rivals in check (no doubt leaving a trail of bloody corpses behind) has not been paying attention.