Or they could just adjust the Winning argument reward to be the exact same as killing all the monsters...
It would even be logical to make the Win reward more XP than the fight as you might lose some stuff dropped by the ennemies.
This. It's not that hard really. In D:OS there are no respawning enemies. Each encounter is designed manually with a predefined amount of enemies. Why not giving the same amount of XP to both winning an argument or losing it?
