The one defining advantage of a republic is peaceful and lawful transfer of power, and limits of the exercise of that power. Any fundamental change requires legitimization via a durable political coalition. While you may be better off under an enlightened king, what happens after his lifetime? In a monarchy a new king can be weak or crueler and nothing can be done short of bloody revolution. In a direct democracy 51% of the electorate can try to strip 49% of their rights. Republics avoid a single point of failure.