A word starts out as neutral, but gradually transitions to becoming a slur (a process that seemingly takes decades), and is replaced by a new politically correct word which is seen as too avant-garde at first, but gets established in part due to a lack of push-back (because any criticism will make the person levelling it seem a bigot/racist/ass), and eventually that becomes the default word, until it starts sliding into slur territory and the cycle starts anew.
I'm not sure if this process can actually be prevented.