If you owe 10 trillion dollars to foreigners, but have a total net worth of 100 trillion dollars, it is other people who have a problem, not you. That's the case with the USA now. (Remember GDP is NOT net worth -- GDP measures the sum total of annual transactions, it is different from net asset value).
The only problem you have is that the 100 trillion is in private hands, and how to get it from private hands into public coffers? Taxation is one way. Inflation is more indirect, and more devastating to the lower income, but more politically acceptable in the long run. Since foreigners own the debt, the bestest way is via currency depreciation. That's why I own gold and gold stocks.
Afghanistan is no issue. How can it ever be an issue when the main fighting was done by local insurgents and not US forces? It was Iraq that was the major policy plunder and diverted crucial resources away from areas that desperately needed them. Because the americans were bogged down in Iraq, they ignored the re-emergence of russia, and this will come back to haunt them in 2015-2030.
The other factors you mention are insignificant. Technologically, USA will always be the world leader. In fact, moving away from being God centered will be a plus for the USA, as it means it can pursue stem cell research without hinderence. But I won't count on that happening anytime soon. Religion is like opium.