Comment: "the peninsula probably would now be one unified, democratic country, like Germany"  - The Globe -

That's exactly what China doesn't want to see happen. All the countries that China shares a border in Asia, are all despotic poor countries that China can influence and control. A united, strong democratic, first world developed country of 70 million people backed by United States ally on the front step of China would make China very uncomfortable.  Millions of illegals from China will also try to get in, like how Mexicans used to in the US border.