If the US was not concerned about NK possibly making a ,ilitary venture into the South, then the US would not be moving it's first rate fighters into Japan and ensuring that Guided Missile Cruisers, DDG's, and FFG's are close by.
Anyone saying that the NK's are not a serious threat to the safety of South Korea and the region in general is only fooling themselves. NK, with it's current leader, will become more volatile the less food and resources they have. The runt will start a war before he loses power, that's a given.