His health was stable and he'd likely make it fine to 70. Making it beyond that is another matter. He was only as philandering as most of the Washington elite, and his affairs would not have come out during his presidency any more so than Lyndon Johnson's did. The media did not dig that dirt until the 1970s, by which time Kennedy would be many years out of office. The eventual collapse of South Vietnam is another matter, but it is important to keep in mind that over 70% of the public did not pay attention to Vietnam as of the first time opinion polls were taken in 1964. At the time, it was a far off situation which was not an American conflict, though we supported the government. It was a rather different situation than after 1965, or even in 1964.