The saddest reality of Mushoku Tensei is that when the final war against Laplace and Hitogami actually happens, Rudeus won't be there.
His children and grandchildren will be thrown into a terrifying apocalyptic war created by their father's boss, while the man who set it all in motion is just a quiet grave under a tree in Ranoa.
Rudeus Greyrat's entire life—his marriages, his kids, his fights with Orsted, his entire 70-year existence—was literally just collateral damage from a unnamed Miko girl rewinding time because she had a crush on a random Japanese high schooler.
Rudeus wasn't the main character of the world; he was just an unexpected bug in someone else's tragic love story.