Conspiracies have several variables, from feasibility in general, to number of collaborators, to motives, to methods of discovery.

Assassinating a popular, progressive president in the 1960s was very doable and didn't take a lot of people to pull off. It will never be settled as to the true motive of the killing.

Compare this to "the moon landing was faked" or "9/11 towers were not only an inside job, but imploded" and they are much less plausible.