Well if Nixon said that he will remain in Vietnam for indefinite period of time ( even with smaller forces ) it could be...
It wouldn't go down like that, I never sad Nixon announces he will stay forever, nor announces a half million troops there forever, a somewhat slower draw down doesn't equate to any of that. And Nixon still in 1972 wins by a landslide because the democrats badly over read the publics actual mood in the early 70s on Vietnam.
The public wanted peace there yes, but not peace if it came with America being driven out the way this op suggests. Frankly Nixon would never do it, because the public would go wild to see a 1969 Dunkirk.
Once you pull out all the troops of course so too do does Congressional interest in funding the country you are allied to as does air support and diplomatic support... see Iraq 2011 to mid 2014. If 1975 came and went with a few thousand troops left in South Vietnam to call in air strikes there is little doubt in my mind there would be a South Vietnam today.