With the intervening of the Celestials, tens of billions of lives would be saved. All that it cost was the death of those involved, along with the destruction of their star as they also served to send a message that would discontinue any more thoughts of chaos and disorder!
So why should they not move to save so many beings? Should they let tens of billions die?
Noah actually did not find an answer to the Grandmaster's question, his mind going towards the Lost World yet again as he wondered- what exactly did the Celestials foresee with this world and its World Engines that they passed the verdict for its destruction?
It was only a Large World that was not that well known, so it didn't serve to send a message to anyone important. He was also moving in this world, which made him wonder if the fate they discerned had something to do with him as well!