Isn't Kennedy as likely to die of a disease as Johnson was OTL? His Addison's Disease might be made worse by the great stress he had trying to regulate the Vietnam War and all of the other chaotic issues of the 1960s.
Johnson let himself go after he left the presidency. He used to smoke regularly, but gave it up on his doctor's advice after he had a heart attack while Senate Majority Leader in 1955. Then, on the flight out of Washington back to Texas after Nixon's inauguration (so literally the very first thing he does after leaving the presidency) he lights up a cigarette and resumes his heavy smoking habit.