The biggest "what if" surrounding the JFK assassination is what would have happened in Vietnam. Many folks believe that it wouldn't have become the abyss that it did had JFK lived. In fact, some believe (erroneously) that his assassination was carried out by individuals in the military/CIA to escalate conflicts in places like Vietnam and Cuba. Of course all of that is purely speculative. JFK was subject to the same political pressures as LBJ or anyone else at the time. He didn't want to be accused of being soft on communism. He won the 1960 election, in part, by alleging that the US had a missile gap with Russia. Something he knew was a falsehood. Nixon was unable to rebut this bogus claim because the information was classified. And LBJ was able to make enormous progress with civil rights. Something that JFK had little interest or ability to accomplish. So these things cut many ways.