I mostly agree with your general point, but the US wouldn't default if China stopped buying US debt. The numbers are a little out of date, but in December 2021, China held $1.1 trillion out of $23.1 trillion in US debt, or just under 5%. [0]
There's extremely high demand for US debt as it's considered the safest bond in the world. All that would happen is interest rates might go up a bit, Congress raising taxes to make up a shortfall, or perhaps some kind of Fed intervention. Default is out of the question.