Kronos had a back up plan. One to be only used as a last hope. As the Titan of Time it shouldn't be that hard to send someone back in time. What it Luke was that person? He would do anything to save his best friend. To stop her from becoming a pine tree in the first place. That one action changes everyone's fate. Would Percy still be the child of the prophecy? Or would Thalia being older? Would she still join the Hunters? Would Luke even go to Kronos? No one knows for sure. Because Luke made a choice. The choice to Rewrite It All.