Counterfactuals are dodgy and you're leaving out most of the details.

What you mean is, "If the Pentagon had been composed of entirely different people and had an entirely different organizational structure then they would have won."

I could have been a star in the NBA!, if I had lived in the United States, been eight inches taller, and cared about basketball.

After WW2, there were twenty years with a trillion 1950s dollars poured into the military and no real accountability or reform effort at all.

By the start of the Vietnam War, the entirely US military was institutionally broken from top to bottom. That trillion dollars had given them the greatest technological military machine in the world, and some of the most incompetent management. There was no war they could really win, as the next sixty years showed pretty decisively (except for some fairly succesful ventures with the UN).

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Of course, I agree with everything else.

I strongly recommend Barbara Tuchman's The March of Folly on this matter!

In fact, I just recommend that book to anyone who likes history because it examines four fascinating case studies in folly, which she carefully defines as not just a mistake, because at least one side in each war is "mistaken" in that way, but...

OK, just read it already. :-D