Key phrase "... if we decided South Korea meant nothing to us."  Focus on "if we".  Meaning hypothetical.  The individual reader can now insert what one wants to try as a thought experiment.

I'll humor you and I don't for a moment think you really believe what you wrote, but here is the bottom line for me regarding South Korea.  South Korea is an ally and  friend.  It is democratic, free, and capitalistic.  We need more nations like it in the world, not less.  We fought with the Koreans in Korea, they fought with us in Vietnam and to a very (almost nil) limited extent in Afghanistan.  But I don't think we need such a large military presence in South Korea as we have and South Korea is not a vital interest for us.  But we can't say that because it forecasts our possible future resolve in defending South Korea.  Much Like Taiwan.   The answer to the North/South Korean problem is more with China than us.