The timetable for the phased withdrawal of most (but not all) troops, that 1965 date in particular, was simply pie-in-the-sky, a fantasy, a made up number. It was added for political purposes: either to pressure Diem and/or to appease those members of Congress and the public who were worried about a quagmire, who wanted an endgame. There was no way for the White House to predict something as complex as the future of Vietnam. And the withdrawal was contingent on events on the ground anyway: the plan says the US would withdraw as the South Vietnamese were able to take on the "essential functions" [however defined] that were being done by US personnel. Two things were going to happen; not just one.
Back to the 1965 date: where did it come from? This is from the Reeves' book on JFK's presidency, "Profile of Power." Remember, the McNamara/Taylor Report was the basis for the announced policy that JFK called for.
"McNamara, Taylor, and [a State Department official in Saigon] William Sullivan began working on the final wording of the report even before they left Saigon. They worked on separate sections, then exchanged rough drafts. When Sullivan saw Taylor’s draft on U.S. troop strength on the flight back, he jumped up to look for McNamara.
“I can’t buy this. This is totally unrealistic,” Sullivan told the Secretary. McNamara and Taylor had put in a timetable: The United States would begin withdrawing troops before the end of 1963, have all troops out of the northern regions of South Vietnam by the end of 1964, and have everybody home before the end of 1965. “We’re not going to get troops out in ’65. We mustn’t submit anything as phony as this to the President.” “You’re right,” McNamara said. He talked to Taylor and they took out specific numbers and dates. But Taylor said he still thought the timetable was a good idea and a good way of putting visible pressure on Diem to shape up or else. “Goddammit,” Taylor said. “We’ve got to make these people put their noses to the wheel—or the grindstone or whatever. If we don’t give them some indication that we’re going to get out sometime, they’re just going to be leaning on us forever. So that’s why I had it in there.” “I understand that,” Sullivan said. “But it’s not honest. If it becomes public, people will know it’s a fraud.”
It was a projection and not a hard date; and done for political reasons. Maybe JFK *was* going to withdraw in 1965; but nobody knew what he was going to do at that time, in late 1963. And nobody included JFK.
The one critical person who did want the timetable inserted, after initially opposing it, was JFK. He was persuaded by McNamara; although it's an open question whether that was needed. JFK was running for re-election and he knew he needed the timetable to persuade Congress, e.g., Fulbright, and the skeptical public. Once Diem was overthrown and the whole situation in South Vietnam thrown into the air, the timetable was revealed to anyone who believed it as meaningless. Well, revealed to everyone who isn't a conspiracist who think JFK was killed because he was going to abandon - not leave, abandon - South Vietnam.