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Ope Ope No Mi is able to grant one eternal life in exchange for devil fruit user’s life.
I think that this ability had been used when devil fruit 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 One Piece 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.
Imagine, Gorosei with bodies of 20-30 years old men, with another 50-60 in years extra added to their life. It’s quite tempting and they will have enough time to find another Ope Ope No Mi user and do the same thing again. So what if they are the same people with different bodies for centuries? They maybe even from Void Century.
*Theory by stefan999