There will always be a debate on what the characters were on the island: human or spirits (souls).
But a subset of this debate can be the theory that the main characters did not know they were ghosts or spirits, so they lived a continuation of their lives as humans. Another tangent would be that they were spirits but were on the island to redeem their souls to become human once more.
The latter would be an interesting concept. One of the themes were "second chances." What if there newly deceased had a second chance to reclaim their human life?
How would one go about reclaiming one's humanity?
If you were an evil person, would you have to do something "good" as in sacrificing yourself to save another?
But if you were a normal person, would you have to do something different, to erase a haunting "regret" in order to be saved?
The island whispers were said to have been trapped souls, such as Michael, after he died. Trapped souls infer that the island was some kind of purgatory where spirits could not move on unless something changed or they were released from their bondage.
If the island was merely a series of "humanity tests," who were the people that passed - - - and left the island as humans? It is noteworthy that Ben and the Dharma group both had a keen interest in island pregnancies. What is human life more than a fetus? It is the start of human life. But children could not be born on the island because their mothers were not human. But the experiments tried to create humans from ghosts in order to find the key to transform back into their human bodies.
Frank survived the submarine explosion and was found by Alpert and Miles, who had decided to continue as planned without Ben who had joined up with Flocke. Frank suggested that
they escape the Island in the Ajira plane instead of destroying it. Upon
reaching the plane, they repaired its broken windshield and damaged
hydraulic systems. This brought the plane into good working condition,
allowing Frank to prepare for takeoff. As the plane was taxiing down
the collapsing Hydra Island runway, it managed to slow down so that
Ford, Austen, and Claire could be pulled aboard. The Ajira plane safely took off just as the
runway began to crack; its occupants managed to escape the initially
Who were the final survivors? Frank was an alcoholic pilot who should have died in the original 815 crash. Miles was a mystic con man who befriended Sawyer. Sawyer was a vengeful con man who killed the man who ruined his life. Kate was a troublemaker who killed her father and fled from responsibility and justice. Claire was bad daughter (her negligence killed her mother) and bad mother (she abandoned her son and went crazy). None of these characters had any major revelations or changes in their personality or morals to deserve to be "re-born" as humans to travel back to their past lives.
Jack was the one who sacrificed himself so the others could flee the island. He took on the guardian role to defeat Flocke (even though it was Kate's bullet that downed the smoke monster human form.)
The Ajira survivors all had deep rooted mental issues tied to self-esteem problems from events early in their lives. They had a sense of abandonment by one parent; they had family secrets which made their skin crawl. They did not want to take full responsibility for their actions. They wanted to escape in their own fantasy image of themselves.
But everyone dreams about their perfect self. Hurley wanted to be a confident, witty, popular and successful businessman with a charming wife and adoring family. But in the end, he did not achieve that self-image. The same is true with Locke. He also had a strong longing to have a sense of "family" but he had a hard time gathering the trust of even friendships. In the end, he was alone in the church. He never did find the family he was looking for through all the hardships of the island.
What was the greatest "asset" the Ajira survivors maintain during their island ordeals to gather their ticket home? Frank, Miles, Kate, Claire and Sawyer all kept away from making leadership decisions. They were soldiers not generals. They did not want to seize power or control. That fit into their plan of self-survival - - - but in reality, the thing that tied them all together was being selfish. They generally lacked consideration for others; they were concerned chiefly with their own personal profit, pleasure or safety.
If true, then "self-sacrifice" was a death sentence on the island. It would be the opposite of common sense or a normal story trope. If only the selfish survived, that would be a bad moral to the story.
But LOST was never really about morals. Characters did dubious things for strange reasons.
If Frank, Miles, Kate, Claire, Alpert and Sawyer were ghosts who passed the island test to regain their humanity, what actually was the test?
They all did survive the judgment (and destruction) by the smoke monster(s). None of them really wanted to seize the island's magic powers away from the guardian. Some of them did kill other people while on the island so a good-evil, right or wrong judgment seemed not to apply. But all of them really had no one waiting for them when they returned home (except for Claire and her son who may be of the age to know his mother had abandoned him in order to reject her). The LOST main story could have been told without any of these five characters. So why did they get special treatment at the climax of the show?
Unless they were always humans trapped in a spirit world. All the other characters on the island were spirits. We know Alpert, who left with them, was a spirit. He became immortal by the gift (or curse) of the guardian to serve him as a go-between with the people he brought to the island. But once Alpert cleared the confines of the island, he began to age. He regained his humanity not by serving the new island guardian or fighting for the black smoke monster - - - he got lucky enough to find himself with an opportunity to leave the island.
And this is why LOST will always have mixed commentary and two sides to any issue. There was no clarity in character traits and story line answers. The ambiguity weaved throughout the series fed the imaginations of the viewers to the point where fan theories were more canon than the show runner's scripts. The stories themselves cast ghosts into the mythology of the series.
The idea that no one survived Flight 815 break up and crash was probable. The idea that the deceased souls could not pass over in the after life because of unresolved personal issues was plausible. The idea that the island ghosts could have a chance to reclaim their humanity is possible.