@Dave,
Disagreeing with you only around the edges.
The lack of a caliphate reduces enemy capabilities and renders them harmless. The second half of that sentence is to strong. You claim our effort “reduces enemy capabilities” and it clearly does but your following clause that it “renders them harmless” goes to far. This threat will remain for a long time. All we can do for the present is render them less apocalyptically lethal. To render them harmless would take three generations of supervised reeducation to cleanse the drive to enforce submission out of their culture.

Regarding Vietnam, it is hard to prove a negative. Would the Soviets have been more dangerous if the US had simply ignored the expansion of Communism in the late 1950s and early 1960s? Probably so. No President could have looked at a map and simply shrugged as the sea lanes between Japan and Singapore appeared threatened. What would have been the most efficient method of getting to a system no worse than the present is another question.