Haki hadn’t really been introduced back when the Yami Yami no Mi was introduced. With Haki, it’s pretty easy to punch an otherwise intangible Devil Fruit user. SO why the Yami Yami no Mi? Blackbeard even admitted this ability was good enough to put up with its own weakness of being unable to avoid attacks like a logia user AND taking even more damage from attacks.

I don’t know about the general fandom, but I just assumed it would eventually be revealed that the Yami Yami no Mi was key to Blackbeard being able to steal devil fruits. Then the value of the Yami Yami no Mi would be apparent again.

But now… we have “immortal” characters. The majority of the Five Elders have shown the mysterious ability to fully regenerate from any wounds received, no matter how deadly they would have been otherwise. For example, Ju Peter was able to recover from having his head chopped off. As such, Luffy, himself an awakened Mythical Zoan Devil Fruit user with enhanced stamina and recovery rate, has claimed that the Elders are virtually immortal, with veteran pirate captain Brogy further noting that he is unaware of any race or power that is capable of such a feat.

The most reasonable explanation we have for this immortality is the surgery that’s preformed by the Ope Ope no Mi fruit user. The fact that it was brought up again by Ivanakov, I think, validates that this is a real thing in the One Piece world and will come up in the narrative.

Well, in short, if this “immortality” was given by a devil fruit power, it can be negated by the Yami Yami no Mi possibly. I think Blackbeard has the ability to kill the Gorosei.