The First World War was the second most recent major war in American history, and I think the statute of limitations has passed on making V-J Day a national holiday.
Veterans Day/Armistice Day is also one of the few secular national holidays that we share with most Western countries.
Total deaths of the Vietnam War, on all sides, were around 1.5 million. Korea was around 1.2 million.
WWII was 50-90 million and WWI was 40 million. Vietnam and Korea (two conflicts that my father actually fought in) were wars, for sure, but not major wars compared to the World Wars.
US had 418,500 WWII deaths, and Korea had 128,650. Both within the same order of magnitude of each other. Vietnam surprisingly had only 60,000 US deaths - it is more notable for the resistance at home it spawned.
There's no reasonable way to conclude these were not "major" wars in the context of US history.
In the context of US history, the World Wars transformed the United States from a parochial, isolationist regional power into a powerful superpower.
The Korean War transformed the United States from a country that occupied half of Korea with the permission of a local, democratically elected government to a country that occupies half of Korea with the permission of a local, democratically elected government. Vietnam transformed the United States into a slightly less interventionist country for 20-30 years and reduced the scope of Johnson's social welfare reforms.
The World Wars also saw the wide-scale mobilization of the entire nation, including rationing, large-scale transformation of American industry to war production, coastal blackouts at night to protect shipping traffic, ubiquitous propaganda, and other measures. World War II was more disruptive to the life of the typical American at home than any other war that didn't physically take place on American soil. (In fact, parts of World War II did take place on American soil.) Korea and Vietnam--"major" wars maybe in isolation, but not at all compared to WWII.
I never argued about the relative importance of the wars, just that your bar for "major" is extremely high.
You also forget about Korea as the start of the communist domino theory that directly led to the Vietnam intervention. And vastly understate the cultural changes that happened as a result of the Vietnam war protests.
Korea and Vietnam simply weren’t as significant as the Revolutionary War, the War of 1812, the Mexican War, the Civil War, the Spanish-American War, the First World War, or the Second World War. They weren’t completely insignificant historical events, but they also just aren’t comparable to the half dozen biggest wars in US history, and that’s what I call “major wars” because the marginal difference between WWII and Korea is so much bigger than the marginal difference between Korea and, say, Grenada.
Veterans Day/Armistice Day is also one of the few secular national holidays that we share with most Western countries.