It's a series where the author pull shit outta his ass at the last minute. Elders immortality gotta have a weakness, it's all depend if Oda wants to hint to it now.
Every arc you got people believe x antagonist cannot fall because they don't see how. I remember having discussion back in Dressrosa day because Doflamingo was considered too strong to fall against Luffy even though it was literally his base island lmao.
It doesn't seem like Saturn has any future in the story.
>all the threat factor he had in his introduction is basically gone, after getting stalled by Nami & co, he was literally treated as a training tool by Luffy for Bonney to train her punch, the difference of treatment between him and Topman that made Luffy twerk and run is night and day.
>showed us both his hybrid/awakening and full beast form, outside Nasjuro Oda only showed the full beast form of other elders (plan to do it later as they still have a role)
>Devon took his appearance
>Kuma and Bonney, main drama of the arc, got their cathartic pay off, Kuma even got to witness his daughter turn into Nika doing so
>Somehow the only elder left on Egghead and chances of him being as isolated as he is now are extremely low
I do agree that the immortality is a huge issue and this is the sole reason why I'm not 100% in Saturn (that and Straw Hats leaving the island) falling but if it wasn't for that everything is lining up for Egghead to be it for him