Is it possible that the Final Shape is the Witness' end? I think it would make sense when it comes to the Sword Logic. Is it possible that the conquest of the Witness is just a search for someone strong enough and brave enough to challenge him and kill him? Kind of like the Ebony Warrior from Skyrim.
Maybe spending eons as a immortal in a mortal realm has made the Witness suicidal and he just wants it to end.
Edit: Maybe that's why the Witness holds such a grudge against the Traveler. The Witness knows that the Traveler can defeat him but just refuses to land the killing blow.
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I'd rather that be Xivu Arath's goal more than the Witness's tbh. Xivu is a warrior so a warrior's death would be the honorable way to go.