The issue is that the global diet for treasuries artificially holds down interest rates. If purchases slowed or holdings were sold, interest rates would rise. Whether it would be a crisis or merely hurt is a marginal distinction.

The government of the USA would go bankrupt in months if interest rates rose to 5%, which is an historically normal figure.

You can sell bonds.

It's rather tricky to predict how a Chinese decision to appreciate the rmb would play out in practice. Many export powerhouses have had very strong currencies for long periods. It's not as simple as saying US exports would go up and the Chinese economy would suffer. You have to consider that the Chinese have savings and would find themselves far richer in global commodities. You also have to consider the US economy's heavy dependence on freakishly low interest rates.

It would be a crisis so long as China remains an export-driven manufacturing-based economy. But, as China becomes wealthier, knowledge-based economy, it won't be as big a deal. With 1/6th of the world's population, I would think there would be plenty of domestic buyers.