China is starting to have labor problems of their own. My brother collects die-cast cars as a hobby. They are mostly made in China. Right now all the major companies who make them are having trouble getting parts from the factories. The factories can't get enough workers at wages which would allow them to deliver the product at the price the customers want. Make no mistake, the cost of manufacturing in China will rise relative to maufacturing in the US.
After their labor problems, China is an environmental disaster waiting to happen. Most of their expansion was fuel by coal and oil. Their cities are often so polluted that people cover their faces with masks. Eventually this will take a toll on the population's health. Since China has socialized medicine, that will cost them big time.
The next big problem is the emerging trend towards more individual freedom in China. We in the West are mistaken to think down the road China will become a Western-style democracy. There is little desire for that type of government. On some levels, central planning and the ability to pass laws without regard to how popular they are is a competitive advantage. I doubt this will change in the long run. However, people will only tolerate so much control of their lives. China already had to relax the laws limiting capitalism in the 1980s. Eventually they will no longer be able to do things like relocate entire cities to build dams, etc. This will severely limit China's ability to expand further. In the end, the Chinese will have most of the freedoms which matter, such as freedom to own property, run a business, live where they want, etc. With these freedoms will eventually come the excess and decadence which has been the downfall of every superpower, including the United States.
Bottom line-China will be top if it isn't already, but its dominance will hardly be supreme. In fact, in the long run this may be good for the US in that it will wake us up, and force us to finally start doing what we need to be competitive. This includes better educating our citizens, rebuilding our neglected infrastructure, and reinstilling the work ethic in our youth. It may also make us discard the worst of our traits, such as the mindless drive towards individual freedom regardless of the costs to others. That in turn will reduce our wasteful ways, once again making the US more competitive. The US didn't win WWII and dominate the economy in the second half of the 20th century because of our rugged individualism. Rather, it was shared sacrifice during WWII, and working together building up infrastructure afterwards, which enabled this. These traits can be reawakened again.
The dark horse in all this? In my opinion it will be how fast we deplete essential resources before we learn (both China and the US) to have sustainable economies. A resource war could cost both sides heavily.