"He who does not think himself worth saving from poverty and ignorance by his own efforts, will hardly be thought worth the efforts of anybody else." -- Frederick Douglass, Self-Made Men (1872)
It seems to me that population alone means that the US will at some point will fall to the third largest economy, but the thing is, it is likely to remain there for the foreseeable future. While India will likely overtake China as the world's most populous country in the coming decades (China's population is actually supposed to decline), no other nation is likely to supplant the United States and take the number 3 position in terms of population. The next closest nations are Indonesia, Nigeria, Pakistan, Brazil, Bangladesh (not in order, now or in the future, but these are the next most populous countries that are also not projected to shrink, which is the case with Russia and Japan), none of which will surpass 400 million people over the next decades, as the US is expected to do (though under what assumptions I am not sure). This fact, along with what, to me, is the doubtful notion that other nations will overtake US per capita wealth anytime soon means that the US will still be a dominant economy, pretty much no matter what.
From a non-economic view however, this is likely to have political implications. In honesty, I am not particularly concerned about India as an economic and political giant. I support building ties with the country because, though I admit my limited knowledge on this subject, I do view them as natural allies, and in general as an open and welcoming society, barring the Hindu nationalist elements. In contrast, China as it is at present has a sense of historical cultural superiority, and is a relatively closed society, combined with a good dose of ethnocentrism (I believe this is argued in a book that was published not too long ago, "When China Rules the World"). This seems like it will present a distinct challenge in the coming years.
I think it would be in the United States interest to promote manufacturing at home, and when job are outsourced, it would be better to consider countries such as India, Vietnam, Indonesia, Thailand, and South Korea in Asia, more stable regions in Sub-Saharan Africa, and Mexico.
Additionally, I think that the US should be encouraging immigration, though moving away from a multicultural paradigm, and more toward the more traditional "melting pot". In particular, for reason that I will not dwell on here, I think that it would be good to increase immigration from Eastern Europe (in particular Russia), Sub-Saharan Africa, and to a lesser extent South and East Asia. In addition, it would be good to continue to be open to Latin American immigration, though diversified away from Mexico (currently ~2/3 Latin American immigrants) and relatively, though slightly, less overall than has been the case in the past.
I think another thing we need to stop, though, is trying to be the world's superpower. Europe has a responsibility in this matter, and the rise of India and China will make competing in their sphere a massively costly undertaking. Vying for dominance against these two won't be like competing with communists. Russia will probably bounce back at some point as well.
We can save ourselves a lot of resources if we accept this and begin to gradually pull back sooner rather than later. Maybe even keep from going bankrupt.
You can never be safe from a government that can keep you completely safe from each other and the world. You must choose.
I agree that the United States will not be able to continue to act as it has in the past, though it may well continue to constitue a superpower. It would be wise to cultivate alliances in various regions to secure global stability as well as maintain a preemminence within the international system.
"This Administration will constantly strive to promote an ownership society in America. We want more people owning their own home. It is in our national interest that more people own their own home. After all, if you own your own home, you have a vital stake in the future of our country."" GWB