A war in North Korea will kill hundreds of thousands of allied soldiers, millions of civilians, and be counterintuitive to US foreign policy and push the reunified Korea towards China instead of the US.
South Koreans, who already view US influence negatively, with loathe the US and Trump for allowing a war to explode which will likely level their capital city and kill millions of their civilians. Without North Korea, South Koreans would've kicked Americans out of the country years ago. A reunified Korea would also add the 25 million nationalist, China sympathizing, anti-US North Koreans into their political and economic mix. The North Koreans post war will encourage cooperation with China against US and Japanese influence and militarization and this will synthesize with pre-existing Southern sentiment and cause Korea to push away the US in as little as a decade after the war.