There is one issue you are neglecting; the USA's terrible education system that is continuing to get worse, and the large influence religion has on the state (which is the cause of the terrible education system). Within 50 years these people will be running the country, and that could lead to some serious problems if the quality of education reflects their future intelligence. The country has a continually growing deficit, and if the next generation isn't too bright for aforementioned reasons, it's quite possible they'll never recover.
Then you have China's continued investment into USA's private sector, and the private sector's influence on the government, you could have an economic takeover of the entire country.
Also, I highly disagree with USA being the scientific and technological powerhouse, Japan and parts of Europe have them beat in that respect. Also, I wouldn't rely on those allies you think you have, most of them rely on China a much larger amount for trade, and if a war were to start between the two, they would more likely support China.
But I suppose I'll get to see what happens. I should quite easily last 50 years.