"South Korea, Japan and the U.S. are three of their largest trading partners. It'd be silly for them to jepordize that."

You obviously have never read any history. Governments do "silly" things all the time.

Hey, Hitler was going to stop with the Sudatenland... Czechoslovakia... Poland... Gee, where did he stop?

If I am wrong, and we have taken precautions/made plans against NK attacking the south, what is the cost, in dollars and lives?

If you are wrong, and we take no precautions and end up with a war with China over Taiwan, and no ability to use Japanese bases because of nuclear blackmail by NK, how many people will die, and what will that cost us?