Psychic is only not that good because of other psychics and chansey. 2-4 pokemon on a team are on it because of psychic. Either to use it or to wall it. At the same time I'd say body slam is the most threatening move in the meta because hardly anything wants to switch into it. Psychic is much easier to switch into. The paralysis chance on body slam makes pokemon a lot more fearful to come in on it, and pretty much everything that runs body slam except lapras can nail gengar with another move.