What I would advocate, which is what I did with Torment: Tides of Numinara (or something like that), was that mobs do respawn unless certain goals are met. My example was that of a giant wasp hive. Until you kill the hive the wasps should respawn. The idea that mobs don't come back is ridiculous.
Another example I used were highway robbers. Sure you killed those bastard last time you came through, but unless you helped the the city watch in this one quest they don't have the men to patrol the roads and another group of robbers take the place of the ones you killed.
If you worry about people farming them, then why not have a flag on the mob indicating how many times its been killed. Each time the XP and gold return is diminished (potentially to zero after just one kill).
This adds to a living world that you have an impact on through your quests, and not just through slaughtering mobs whole-sale.
