The Korean War is universally and intentionally ignored in such discussions, because to bring it up as an example of a needless war when eg discussing Vietnam is to proclaim the South Koreans should be living like the North Koreans right now. That is, it's to say that they should be enslaved under Communism and living at the very bottom of third-world style conditions. So nobody will dare to say that, they'll just ignore the Korea outcome and focus on Vietnam as the example instead.