Whatever trait supposedly justifies eating animals, some humans lack it too
People often justify treating animals as lesser by pointing to traits they lack, like advanced reasoning or self-awareness. The argument from marginal cases replies that infants and people with severe cognitive disabilities also lack those same traits, yet we don't think that justifies treating them as lesser. If having the trait isn't really what grounds a human's moral status, the argument goes, it can't coherently be the thing that excludes animals from moral status either.