Repeating the last line of dialogue is not just a way to indicate that there's no more dialog, it often also works as a remainder, giving you the most important kernel of information ("You should go to [place] and talk with [npc]"), in case you come some time later and forgot what you were supposed to do. You can indicate there's no more dialog in many ways, but you'd lose that secondary feature. Same thing if the NPC just keeps babbling generated drivel.
So true. In such an LLM-driven game though, I would imagine the player would just ask the NPC: "I forgot what to do" or even "Can you explain it in other terms?" (if the quest description isn't clear enough).