Your facts are a bit sketchy. China and Japan both hold about 1 trillion in US debt, but it is acknowledged widely that China is the largest foreign holder of US debt.
Foreigners hold 47% of US debt while Americans hold 53%. Not such a wide margin - 6% in the grand scheme of things isn't much.
Not sure how my facts are sketchy. The governments of China and Japan may each hold around 1 T each but Japan as a whole holds more US debt. Again, US citizens are by far the majority holder of US debt. Closest is Japan who holds around 13% of that debt.
Anyways, back on topic. China's population is actually going to decrease significantly because the 1 child policy. They will have a large elderly population, many of whom do not have retirement savings of any kind and likely will depend on their child for support. This will cut deeply into China's appeal as a rich target market. Furthermore, with fewer workers, labor costs will go up just when companies are realizing it may be cheaper to on-shore jobs back to the US.