@Robby:
Because he was 100% dealt with and then in cover story his went into the wild again due to a failing of Coby.
It was resolved and then Oda went out of his way to un-resolve it.
It can be dealt with in one panel of Coby apprehending him in an action showoff moment to show how he's grown, he doesn't need to be a major antag with upgraded fruit and haki powers, but that needs to be adressed for Coby's sake.
As for Enel? I'm sure he'll be back but he doesn't NEED to be. The audience got hints of the bigger pciture, and Enel continues off on space adentures. He left the story and achieved his dreamms without any explicit promise to come back… though I guess his assembled army ending is something.
Interesting stance. Personally, I've never thought of it as un-resolved. Maybe if Morgan had made it back into the Marines and was abusing the organisation's authority like he used to, but even after the cover stories he remains discredited and exiled, a criminal to the men he used to lead. He's no more in vital need of tracking down and finishing off than Kuro or any other of the minor villains Luffy's been content to let go over the years because the fact that he's stopped doing the bad thing is more important. If anything, his escape gave Koby and Helmeppo's story's more closure instead of less - Helmeppo got to stand up to him and Koby also got to show what he learned, standing for what he believed and against fellow Marines doing something he thought was wrong. For me, those moments outweigh anything that could come from Morgan still being out there.
Don't get me wrong, I would love a moment showing Koby and Helmeppo bringing him in. But does the story need it? Nah. And there's not much of a place for it in the story now. No matter how significant, it would feel small fry after Koby was assigned to take on Hancock. Maybe in the final, post-series Decks of the World part of the epilogue…
And yeah, we know that the moon ruins and futuristic stuff Enel found has to be addressed in some capacity, with or without Enel. But now that he's tied up in it (and considering how popular he is) it'd be foolish not to bring him back at least in a small role when that happens. I agree that it doesn't have to be big, but in my mind the moon connection makes it feel pretty close to inevitable.