That kind of debt is not something a country can survive. That's $190,425 per US citizen. I'd guess they'll suffer the same fate as the British Empire did after WW2, find themselves unable to support itself and fall out of power. Odds are, it'll be for the same reason too- spending too much money on war. The British empire at least only did it when they really, really had to. America is overly eager to get themselves shot.
I feel like the US is very much already in the decaying phase of its global prominence. With our outrageous national debt (granted, many other countries are in loads of debt too), backed by "faith" currency, collapsing infrastructure, disparaging wealth gap, political impasse and stagnation, civil unrest (Baltimore, Ferguson), and apathetic population, we simply cannot maintain strength at home.
Meanwhile, abroad, the BRICS nations are certainly rising. Russia is fairly unreliable, due to the economic sanctions currently in place, but it's doing its best to have China and Iran dethrone the dollar so that it can be free of our evil currency war. Brazil is most definitely growing, but it's not in the spotlight. Same with South Africa. China and India however, despite China's collapsing stock market currently, are likely to be leaders on future tech like nanotechnology, genetic engineering, quantum computing, and the future of humanity in space. I say that they are likely to be so because the talent drain to the US is slowing and a lot more startups are coming to these two countries. That, and there seems be a general shift towards the East in terms of power going on currently. The renminbi could easily displace the dollar as the king currency, if China can create deals with Iran and Russia, bypassing the dollar. And if they back the renminbi with gold.
All of this along with the fact that global tensions are rising, especially in the Ukraine region and in the East, and a lot of countries are pointing the finger at the US. The wars in the Middle East are seen as pure aggression. And with the NSA information leaked about spying on foreign officials, our popularity overseas is really declining rapidly.
This is a basic summary of why I think the US will fade out of global prominence in the next 25 years. Not collapse, mind you. Just be sort of in the background as a member of many world events, just not at the head of them.
Thanks to the incompetence of our soundbite-spewing "leader", we have been handing over our sovereign territory, with some of our citizens on it, to other countries, experienced some of the worst legislation in history, and made massive concessions to other nations that have no right to be as cool as us. In short, we're in decline because we've stepped away from the morals we were founded on.
The countries (Brazil, Russia, India, China) that could "un-seat" the US in terms of economic power are not even close to making this scenario a reality.
China is the closest to making this a reality, since they are the second largest economy. First, China would have to become a Democracy. The two central ideas here, Communism and Capitalism are two competing ideas that work against each other in pursuit of the same goals. For example, Communism favors government control over property and businesses. Not just control, but ownership. While Capitalism favors private control and ownership. How can each of these two ideals work together toward this same goal? They can't; one will suffocate the other. It's just a matter of time.
Russia, still recovering from the fall of the USSR. Also, oil being their cash cow, is no longer a fail safe plan. Oil prices have fallen much lower with the entry of fracking, and America is now the number one oil producing nation! Oh, and Putin is taking Russia back to the USSR? Putin is more interested in gaining their empire back rather than working toward freeing economic markets and expanding their own economy.
Brazil shows much promise in climbing the economic chain, but they still have many parts of their country without sufficient infrastructure. Much of the country doesn't have roads, power, etc.
India has much of the same issues as Brazil. They are much better off with American off shoring and their dedication to IT. A stable nation. Many companies are setting up corporate offices to reduce costs and employ far cheaper labor. But at a GDP of 2 trillion, they still have a long way to go before even becoming close to the economic power of China let alone the U.S.