I see this thinking all over the place. It's antiquated thinking. You've likely never been to China. I've have been to China multiple times on research missions.
China does not need the dollar. In fact, more than 50% of China's trade with the world today is now priced in yuan, not dollars, meaning countries of the world, including big US trading partners/allies, are more than willing to trade with China in yuan... and China's economy is growing nicely. The country recently hit an export record. Moving away from the dollar has not hurt the country in the slightest.
Less than 3% of the Chinese economy is tied to the US, so China could say "Screw it; we sell nothing more to the US," and China would be fine. It could ramp up business to other nations and push incentives to drive local sales.
China is already replacing the dollar. And it's not alone: Singapore, India, Thailand, Malaysia, Russia, and on and on are all right now building non-dollar trading systems so that they can trade amongst themselves outside of the dollar.
China has built a system with Russia and others in BRICS called The Unit, which is backed 40% by gold and 60% by local currencies (not dollars) and will be used as a trade currency between countries. Not hypothetical, already in pilot phase this year.
China has been dumping US dollars/Treasuries for years to move away from the dollar. It now has unofficial gold reserves that dwarf the US (widely reported and researched, including by me, going back to about 2010). When the dollar dives, China reaps a whirlwind and it doesn't give a shit about the value of the dollar in that moment.
So your basic math is - basically - wrong.