Vietnam War didn’t meant to be that big, McNamara even said that JFK wanted to withdrawn 1000 US forces personnel (MACV (MAAG then) Vietnam, in Indonesia called as MILTAG Indonesia) from Vietnam at the end of 1963. But that withdrawal didn’t mean that they wanted to left South Vietnam just like that, cross border (deniable) operations that was responsibility of CIA to be transferred to US DoD through OPLAN 34A. The point was to made North Vietnamese not in favor of backing insurgency in South Vietnam anymore.
The problem was during the Vietnamization, the RVN Strategic Technical Directorate, those who worked with MACV SOG and did its work after the Vietnamization, instead of cross border operations they put more their effort within South Vietnam, left logistics through Ho Chi Minh trail untouched and North Vietnamese not busy of “problems” within their border, while their war couldn’t be just fought within their country. Yes, the air strikes still needed US not fully South Vietnamese, but didn’t mean they couldn’t get that. Only COIN within South Vietnam was not enough, they needed to be the insurgents.