One of the great unknowns is what happened to the Ajira plane after it took off from the island.
We see it fly over a dying Jack, but we really do not know if it ever made it back to the mainland. It was a damaged plane fixed without tools or supplies. It may have not had enough fuel to reach a destination. And if the island's electromagnetic shell was still working, it would have not let the plane travel in any direction except in circles.
But we are led to believe that the passengers got home safely. That they returned to their lives and died "much later" than Jack, according to sideways Christian in the church anteroom.
But is that believable?
If the Ajira return passengers got back to the mainland, they would have been celebrity-heroes. They would have been hounded by the media for interviews, shows and biographies. They would have met other celebrities, been part of the B-list jet set, and had their lives totally changed.
Frank piloted the plane off the Hydra Island with Kate, Claire, Richard, Sawyer and Miles on board.
One would assume that if Frank got back, he may have retired to Florida to charter smaller planes. It never made sense that he would have returned to a commercial pilot status after the 815 crash psychosis and alcoholism. Frank would have been the type to recede into the background.
Kate would have returned to the mainland as a free woman. But what would have happened to her? She was still married to Florida police officer Kevin Callis. She could not have been declared legally dead. Kevin truly loved and supported Kate, but he must have been devastated when she left him. He never showed up in the O6 arc. Would he ask her for a second chance upon her return to the States? If so, then Kate should have not ended up in the sideways church with Jack. She would have created a long life with Kevin. But that does not seem to be the case, as Kate arrives without him in the afterlife.
More likely, Kate would have reverted back to a life of recklessness. She enjoyed being a fugitive, using her wits to get out of trouble. She was a troublemaker. She tried to change by being a homemaker to Aaron, but she abandoned that notion on the wild dream that she could find and return Claire to him.
Likewise, Sawyer would have come back with no career, no prospects except the con game. But he also had a person to re-connect with: Cassidy, the mother of his child, Clementine. If Sawyer had changed into a responsible adult during his time with Juliet in the time loop 3 years in the Dharma camp, one could imagine Sawyer wanting to have a "real" family and a "real" life. But that does not seem to have happened, because Sawyer does not end up in the afterlife with either Cassidy or Clementine.
Claire's return would have been problematic. First, she was an emotional and psychological nightmare. She loved a dead squirrel baby. If she returned to the mainland, she would have had to go back to Australia to face her mother in order to re-connect with Aaron. But the main issue with the O6 arc was that Claire's mother, Carole, was on life support because Christian was paying for it. When he died, the support ended. It was a miracle that Carole suddenly recovered and found her way to LA for Christian's funeral. No one would have told her that. And for Carole to take Aaron out of the United States when all the documents said it was Kate's baby - - - again, legally far-fetched and unbelievable. But even if the fairy tale ending happened and Claire was reunited with Aaron - - what would she do? Would she seek out Aaron's father, Thomas, and live happily ever after on her celebrity fame? Doubtful, since it seemed that Thomas was the type not to accept responsibility. Claire's return to the afterlife casts doubts on whether anyone survived the return flight. She was pregnant with Aaron, meaning she never reunited with him. It may mean that her mother and Aaron rejected her - - - so she may have been institutionalized from her island trauma and grief. But how she could re-create the soul of Aaron outside his own time-life line is a major plot hole. (This is also true with Sun and Jin).
Miles return to the States was also problematic. He had given up being a spiritual con-artist, speaking to the dead to give loved ones closure. We don't know if his mother, Lara, was still alive. But if Miles was going to re-start his life, one would expect he would start back in his hometown, Encino, and probably return to a job as a mechanic. The only other alternative would to follow Sawyer down his path if the con game was still an option. But since Miles does not make it to the sideways church reunion, he moved on to his own life.
Richard would have continued to be a lost soul. He was more than a 160 years old when he left the island. Nothing is known about what happened to Richard after he left the island. But the gray hair he discovered before leaving suggests that he later aged as a normal person would. But he had no place to go - - - everyone he knew was long gone, dead. Would he have reconnected with the few remaining remains of Ben's network of spies and assassins? Probably not, because they had no purpose anymore. Richard would return to the modern world with no identity, no resources, no family and no friends. He would truly be a man out of his time. And that would be a sad, troublesome end for him. He also did not show up at the church.
The sideways church reunion was supposed to wrap up the grand question of LOST. Christian reassured Jack that they are all "real," Jack's life was real, and the people in the church were real. Jack asks if everyone else is dead too, and Christian explains that
"everyone dies sometime, kiddo. Some before you, some long after you."
When Jack asks why everyone is here now, Christian responds that "There
is no now . . . here", and that this is a place they all made
together to find one another, because the most important part of Jack's
life was the time he spent with these people, and that's why they are
all here; no one lives life alone. He needed them, and they needed him;
to remember, and to let go. Jack tells Christian that Kate said they were all leaving. Christian explained they aren't leaving; they're moving on.
But if the Ajira plane did return to the mainland, and those passengers did live long lives as expected, one would presume that they would have made new friends, had new relationships, gotten married, had children - - - created a brand new life separate and more important than the island world. If Sawyer spent the next 50 years brooding about losing Juliet, then that would have horrible (and totally out of character). If Kate secretly pined for Jack, that would also be counter to the chances she had with him on the island and during the O6 arc (when she rejected him for the last time). We can then only assume that she led a spinster, quiet and lonely life if Jack was the best thing she ever had. And apparently, Claire had no life since she is re-creating it over again with Charlie.
Which gets us to a new question: is the sideways (after life) itself an illusion, a fantasy, a re-boot? It must be if you take Sun-Jin and Claire's post-pregnancies into account. Then if that is not believable as in a truth, then can we take anything that Christian said as being true? It makes more sense that the Ajira flight did not make it home. That the passengers last moments of life were thinking about the island and the people they left behind.