I think this is a very foolish way to look at things. Autarky is generally, not a good thing. You simply make things more expensive for yourself.
One has to buy food regardless of price. If China needs to import food in a crisis, other countries can charge whatever price they want
Lol, if the US Navy is blockading your ports to starve you, food security is probably not the biggest concern.
I think your outlook is far too pessimistic on Chinese agricultural potential. I believe China is more than capable of being self-sufficient in food even with much larger population than it currently is. But aside from self-sufficiency, there are also a lot of gains to be gained from importing food.
And, I hardly expect US Navy to be capable of blockade Chinese ports anywayIf anything, we (United States) might be getting kicked out of WESTPAC within the next 10 years.
The US navy can attempt to blockade China. Which corporation will be willing to send their millions dollar ships with their millions dollar worth of cargo into active warzones?
The US can simply force none of the world food distributors (4 major ones controls like 90% of world food trade) sell their food to China.