For most of the past 20 years or so, China has been buying dollars on the international currency market with its own currency to keep the yuan devalued and nurture China's booming manufacturing and export sectors. As a consequence of this, China has accumulated trillions of dollars,

Who's holding the trillions of yuan that they sold their dollars for?

Usually, this would be fine, but because Chinese banks have so many dollars this poses a systemic risk to the global economy. If Chinese banks dumped billions or even trillions of dollars simultaneously they could create a dangerous feedback loop whereby their dollar dumping pushes down the value of the dollar, encouraging further dumping, pushing the dollar down further. This would create a rapid depreciation of the dollar with unpredictable consequences for America and the global economy. Essentially, the point I'm making is that China's enormous dollar reserves pose a systemic risk to the world economy, especially given quite how opaque the Chinese banking system has become.

The value of the Dollar against the Yuan, then there's the ever declining usage of dollars for international trade:

In effect, it would suck for America when it's dollars won't buy it shit and that's about it.