Ope Ope No Mi is able to grant one eternal life in exchange for DF user life. I think that this ability had been used when DF user was about to die. The sacrifice wasn’t that big and maybe Ope Ope user was looking for someone to give him last gift. Of course, that ability is extremely useful for almost anyone in OP world. Luffy might declined, but someone else like Doflamingo or Gorosei will be looking for that power. Unfortunately, this technique have one, big flaw. It can be used only on person. For group like Gorosei is unacceptable. So i was thinking about something else. Before revealing ultimate technique Doflamingo mentioned something else – Personality Transplant.
*Theory by stefan999