I tend to be a bit sceptical when I hear that China will continue its rapid ascent into world dominance. The main reasons for my scepticism are:

• As China's GDP continues to grow, the workers of China will begin to demand higher wages and benefits. Just like the Industrial Revolution that happened in North America and Europe at the turn of the last century, society will progress such that working conditions are forced to improve. Someone who lives in a hut and can barely afford fish and rice, making $2 a day assembling 50 inch plasma TV's... will start to wonder why they can't afford electicity while their boss lives like a king.
  

• China's centralized communist government will make a fatal mistake. So far China has been transitioning to a mixed capitalist-communist system fairly well. But it's not too far out of the realm of possibility that one day the communist government will make a decision that goes against its economic interests - like invading Taiwan or Tibet. The same type of problems that plagued the dictatorship in Iraq (invading Kuwait) can befall China, since not all of their priorities are economic. Or the next generation of communist leaders might decide economic reforms went too far and move China back into a wholly communist system - destroying independent business in the process.
  

• Their human rights record can catch up to them. What if another Tianamen Square happens? Or they execute the wrong activist, or do something so outrageous against basic human rights that world opinion turns quickly against them? One communist party leader in one far corner of the country may think firing wildly into a group of protestors with a machine gun is a good idea. But if that gets captured on video tape and broadcast to the world, China might find itself on the outs.
  

• America gets a vote. Obviously there are many things the United States can do to get in China's way on their way to the top. Not all of them are great options, but in desperation America could "repatriate" China's American assets. It could refuse to pay back any money that the U.S. owes to China. It could mandate that American companies sell products only manufactured in America. It could impose huge tarrifs on Chinese imported goods. It could impose sanctions. It could invade China militarily.
  

Obviously most of these are unlikely. But let's not underestimate the fact that America could view China's economic dominance as a "threat to national security" and justify openly or secretly acting against China in its own interest.