Frightened working as RAW will also be more fun for players when used against them. In a turn based game you want to be very carful about taking away a player's agency over their character, and this should be reserved for powerful and rare abilities. There is a big difference between losing your entire turn while watching your character run away, and being restricted to "making attacks at disadvantage while the source of your fear is visible" + "can't willingly move closer to the source of your fear."
This is of course in addition to the balance arguments. A single Battlemaster maneuver shouldn't auto-win combats.
