Actually, it called capitalism. Some soy bean trader is now buying the US beans a little cheaper because there may be an excess in supply. So, that trader now pays a little less and sells them to somebody else who then sells them to China.
China will still buy just as many soybeans as they did before. They will just buy them from a different source. The markets will adjust and somebody else will fill the void.
“China will still buy just as many soybeans as they did before.”
Most of their soybeans go to feed their hogs - they are the world’s largest pork producer. It is their staple meat.
Unrelated to the trade war, their hog population has been devastated by the African Swine Fever virus, and their feed requirements are way down this year.
But otherwise, your principle is correct - commodity products are easy to substitute.