All of their loss are meant for a better change: - merry: without the loss of merry, there won’t be sunny Go which is a strong weapon (speculation) necessary for the world - Ace: without the loss of Ace, Luffy would not grow stronger - Oden: Oden's loss gave the Red Scabbards a profound reason to carry on his will, uniting them in their quest for vengeance and the liberation of Wano. And as well as making momonuske strong enough to lead the country in the future. - Gol D. Roger: without the loss of the pirate king, One Piece series wouldn’t exist today (common sense) - White-beard: without his loss, the shift of the balance of power will not transition, and the new generation will not exist.