pretty sure they could just turn to solar, wind or water for energy (especially wind turbines or water dams can build even any developing country without kuch know how, let alone Japan, no need any rare resources from China for that) if they really wanted/needed to be self sufficient

same with food, it's just not worth for them right now, quite irelevant comparing status quo with situation where you want to be isolated not part of globalized system, just because now they import 60% food doesn't mean they need to import 60% of food