The Chinese economy exceeding that of the U.S. is a near certainty. The question is: When? We don't have much to fear from China, however, even when that happens, for two reasons:If China can maintain a GDP growth rate of above 5% (present is 7.5%), the Chinese economy will overtake the US economy by 2030. This is common knowledge these days within economist circles.
When exactly do you expect the Chinese GNP to exceed America's? 2150?
But if the US economy GDP growth rate increases itself by a significant margin, then yes it will take much longer for the Chinese to catch up, but one day eventually the Chinese economy will exceed the US economy.
1-The U.S., even when it takes second place in economics size, is likely to remain a "superpower" through the end of this century and (though predicting so far is risky) probably the end of the next as well.
2-As China grows an educated middle class will emerge, just as it did in Europe. And, just as in Europe, that middle class will collectively demand a more representative form of government. If we hold our lead for a century or so, when the Chinese catch up, they'll look so much like us it won't matter who is in the lead. (democracies don't go to war with each other.)