how to make the ending make sense is easy, you have to look at the story from a wider point of view if you had the power to fix everything you definitely would is it a cheap trick ?maybe but tbh the world with sovereign and fragments of brilliant light this would be a good enough ending because consider this the whole cycle of war between them was close to an end and it was going to be a disaster if it wasn't for sung and the way he asked to revert time is fitting because humanity was never to pay the cost of the war it was just caught between the crossfire sung chose the option where the world continued without interference so he just saved as much life as it should be , without gates there wouldn't be people who die because of the rulers and without monarchs there wouldn't be an absolute chaos and without them both there wouldn't be someone unfairly overpowered except him which he paid for by solving the war by himself so that evens out