So here are my thoughts as to where our Losties would be if their plane hadn't crashed. (Obviously this is all speculation, so feel free to disagree)
- Jack: His father is dead and his mother blames him. He may still have a practice for now, but it's only a matter of time before his guilt over his father's death and the death of his marriage catch up with him and he spirals into depression, addiction and a jeard. (I stole this term from the Ack Attack. Love it too much not to use it :) The only difference is now he has nothing to snap him out of it, no purpose, no anything.
- Jin and Sun: This could go one of two ways. 1. Both of them continue to be ruled by Paik. They can't have children and they live in misery, growing to hate each other more and more each day. 2. Sun works up her courage and leaves Jin. He now has no purpose. Sun lives semi-happily, but always wishing she could have back the Jin she loved before her father got his claws into him. Jin either leaves Paik a crushed man, or doesn't care any longer and turns into a ruthless follower of Paik.
- Sawyer: He's still out for revenge, still a bitter, angry con-man with no future and no hope. Cassidy hates him. He'll never be able to do much of anything for his daughter. Maybe he'll eventually find the original Sawyer, but probably not. Cooper's too smart for that.
- Kate: She's in custody and after all that's happened, she's going to prison for a very long time. No Aaron, no purpose, no hope. Just a life in jail for trying to protect her mother.
- Locke: He's still stuck in a wheelchair and stuck in a terrible life. Does he even have a job anymore? Was he just taking leave or did he quit in anticipation of what wonderful things would happen on the walkabout? Helen has left him, his father tried to kill him and he has no purpose. Nothing to keep him going. I think it's very possible that he would attempt suicide in short order.
- Sayid: He may never find Nadia. Yes, he's searching for her, but the press over the O6 is what brought them together. He might just end up wandering forever, trying to find his lost love.
- Boone and Shannon: Both still alive, but Shannon's life dream was taken away by her step-mother, Boone is on to her schemes. She's still a self-centered brat and Boone is still panting after her. Maybe not terrible, but not exactly good, either.
- Michael: He's the only one that might have a chance at a decent life. Without all the stress on the Island he and Walt might have been able to slowly get to know each other. If nothing else, he does love his son. It might have been enough.
- Claire: She would have given up Aaron and returned home. That's pretty much it. She'll probably be a little shy of relationships after Thomas left her and will regret having given up her child.
- Charlie: Sure, he died, but with the way he was headed, he probably would have OD'd or something anyway. Here he had a chance to develop good friendships, help Claire and Aaron and then die nobly to save everyone else.
- Hurley: I don't think he was convinced that he's not cursed. Of course, that's not the most terrible thing, but we also don't know how well he owuld have accepted his father's return in the long run. In the flash-forwards it seems that he's grown to trust his dad, but would that have happened without his being lost in the crash?
- Juliet: If the H-bomb is what was causing the infertility then she won't ever need to go to the Island. But will her sister still be healed of the cancer? We're led to believe that something or someone on the Island is what heals her. Juliet might have watched her sister die. Who knows where her life could have gone after that. It could go good or not.
- Libby would still be alive, but she never would have met Hurley. We don't know much about what her life was like, so I don't have much to speculate on.
- Ana Lucia: Her career is over and she doesn't really seem to have any purpose. She would still be alive, but aimlessly drifting.
- Desmond: He still ends up on the Island, but there's no need to push a button now, so what happens to him? Can he get off the Island? Does he get taken by the Others? Does he ever end up with Penny?
Okay, so they're still alive, but for most of them that life is really going to stink. Are they really any better off? Maybe a few of them, but for the great majority, no.
Do I think they'll be able to change the past? Honestly, no. I think the show is set up to much in the "whatever happened, happened" camp. I think whatever they do will end up causing the very things they're trying to avoid. The time travelers are the variables, but they can't change the past. They can only change things that directly involve themselves at that time, not things that involve past events. So they might be able not to all die, as Richard said they would, but they're not going to be able to stop the Incident. JMO, but there it is.