Perhaps if the Hi Chi Minh offensive had succeeded, we might have avoided the massive collapse of the South Vietnamese state through its own incompetence and corruption, and the systematic starvation of so much of the population.
South Vietnam, after 1975 continued to be a corrupt basket case, lurching from failure to failure. Despite many extravagant claims about its military forces, they seldom fought and only rarely fought well. The South Vietnamese economy was entirely dependent upon American aid, and the South Vietnamese elites looked exclusively to that aid rather than to their own country. Land speculation resulted in many South Vietnamese being dispossessed of lands, entire regions of villages were abandoned, with a population residue remaining as landless tenant farmers for cash crops and the cities absorbing large populations of landless migrants. As the local agrarian economy collapsed, and populations shifted, the remnant Vietcong withered on the vine in the country, but found new strength in cities.
The South Vietnamese government responded with new waves of repression and pogroms, but riots, particularly food riots broke out. The South Vietnamese government had the US bombing its own cities.
By 1982, massive numbers of refugees were fleeing South Vietnam for Indonesia, North Vietnam, China, Thailand, Cambodia and any other country which would have them.
After this, the South Vietnamese state essentially ceased to exist in any meaningful sense. The US maintained a peacekeeping function in Saigon, and successfully excluded the North. But by 1987, after the Marine Corps bombing, even Reagan had had enough, and begun phasing out.
It was replaced by a multinational peacekeeping force, including North Vietnam. However, one by one, the peacekeeping forces dropped out. By 1992, with the collapse of communism occupying US attention, North Vietnam emerged as the de facto ruler of the south.
The price for these decades of 'freedom' was a legacy of unending misery and horror, thousands of American lives wasted, hundreds of thousands or millions of dead Vietnamese, and billions of dollars abandoned.