Ask them what they think the single most important factor was in the fall of the Round Table.
I could go on about this for ages and I’d rather just link you to my book… but that would be self-serving. Instead, I’d just casually mention that the fall of the Round Table is the chivalric ideal itself.
After the wars are won, the Table Knights fall into what T.H. White refers to as ‘games mania.’ They have nothing to fight for because the wars are over and the knights inevitably begin to fight each other. The battle against Mordred could have been against anyone. It doesn’t really matter whether it’s Arthur’s inbred son, jealousy amongst the knights, the Lancelot/Guinevere affair–which is of later tradition–or any of that; eventually, someone would prevent the eternal peace of Arthur’s kingdom because we, as humans, are flawed. The chivalric ideal is a concept that cannot be maintained without an outlet for the knights’ inherent violence and the imperfection of human nature.