This move would further alienate the US from the rest of the world. It is a place to conduct diplomacy with the majority of nations, and it is conviently headquartered on our soil. Pulling out would erode our soft power, and we would end up with more millitary conficts. The UN was created after ww2 so we would avoid major nations tearing each other appart. Leaving would be insane, esscially if you were only concidering america's intrests.