The Korean war is more murky. Almost nobody knows it now but South Korea was a military dictatorship, not a paragon of democracy and freedom.
Veterans Day/Armistice Day is also one of the few secular national holidays that we share with most Western countries.
For me the definition of "major war" is were civilians forcibly conscripted.
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.
There's no reasonable way to conclude these were not "major" wars in the context of US history.
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.
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.
"major hostilities of World War I were formally ended at the 11th hour of the 11th day of the 11th month of 1918, when the Armistice with Germany went into effect." https://en.wikipedia.org/wiki/Veterans_Day
So any attempt to 'move' Veteran's Day will likely meet resistance due to this connection. This is also why this day is a fixed calendar day (at least in the US) rather than a "first/second/third Monday of the month" type holiday like most of the other official federal holidays. It shares the same connection with specific exact calendar dates as does the 4th of July, Christmas, and New Year's day.