When the U.S. pulled out, victory along these conditions had been achieved. The Viet Cong were no longer a serious threat and the ARVN, with continued American support, was capable of standing up to and defeating the NVA. The problem came when Congress began to handicap the President's ability to continue this aid, and led directly to 1975.
I think that's kind of like mistaking a snapshot for a movie.
The argument that the Viet Cong were no longer a serious threat ignores the likelihood of the Viet Cong regenerating.
There was simply no meaningful way to interdict the Ho Chi Minh trail. At least, not by South Vietnam. Even the United States using massive air power for bombing and chemical defoliants found itself hard pressed, and the effort destabilized and destroyed two neighboring countries. I don't think that the Trail was ever cut completely by the United States. Certainly interdicting it was out of the question for South Vietnam.
So, with literally an open highway, the Viet Cong could simply continue to access money, supplies, weapons and ammunition indefinitely in open ended quantities, so long as the Soviet Union, China, North Vietnam and other states chose to do so. There's no reason that any of them would just walk away.
So what were the limiting factors for the Viet Cong? Recruitment. Recruitment was in turn driven by corruption and incompetence, both of which the South Vietnamese state had in abundance.
I'm not talking Tyranny, history is full of effective Dictatorships. But the reality was that the South Vietnamese regime under Diem was not effective, nor were its successors. It was an urbanized kleptocracy, Diem was replaced, but that didn't actually change. The South Vietnamese state was unrepresentative, corrupt and rotten to the core. To suggest that the SV military might be competent is very nice. But that alleged competence didn't filter upwards to the state or ruling elite, and without that, the whole house is built on sand.
The likelihood is that even if you butterfly the fall, the odds are that within a few more years, you'd be back to the pre-Tet offensive situation of the South Vietnamese government steadily losing control of the countryside.
By the way, I like the quote - the ARVN
"with continued American support" was capable of standing up to and defeating the NVA. Certainly, after a decade of massive support and investment and the expenditure of vast wealth and resources in trying to prop it up and train it, the ARVN was capable of fighting
"with continued American support." That's pretty much a blanket admission that after a decade of investment and support, it still couldn't handle the job on its own.
So we're seem to stuck with permanent or at least indefinite American support. We're not talking tripwire forces as in South Korea. No, we're talking continuing American support... basically fighting, or bombing indefinitely. Continuing American support until such time as....
And by the way, we're still back at the snapshot vs the movie. In 1972 the ARVN after a decade of hard work could almost stand up all by itself. Could they sustain that high water mark without ongoing, continuous, massive infusions of American money or help? Or were they going to slowly or rapidly degrade? The real question is just how stable was the ARVN and how effective was it at sustaining its level of competence?
I would argue that to sustain its levels of competence, the underlying government and social structure would require substantial reform, and that simply wasn't in evidence. Degradation would be inevitable, and with that you'd once again require increasing levels of American support.
It's possible for the United States to win in Vietnam, but not easy. As weak and battered as the NV were, the perpetual problem was the weakness of the SV, and that was a problem that the United States recognized from the outset but was never able to solve.