Going back I look at the Jack/Ben characters differently.
I do like their stories now.
Both show the importance of faith, not in a religion, but in general. Just the the belief itself, the positive attitude that somehow all will work out.
Ben never has faith. Ben never saw Jacob and never really believed. No matter how much power or people he had behind him he couldn't FORCE the world to give him what he wanted. In the end Ben was the story of a character left with resentment, regret, and without power.
Jack finally found faith. He died with purpose. That sense of purpose, due to his faith in his actions, made HIM feel his life had meaning and worth. He didn't feel that way before. He FOUND his meaning.
Jin and Sun always loved each other but let other things get in the way. They died together because they made that decision. They decided nothing would come between them.
Death is not losing. It's not failing. We all die. It's how we die that we have some choice in. Is it for a purpose? Does it have meaning? Did our lives make others and a small part of the world better?
All the characters were lost before they came to the island. On the island they got a mulligan. Given a 2nd chance some made the most of it and some failed.
In the flash sideways they were all given what they really wanted. Sawyer got all the resources he needed to find the killer. Jack got a happy family life and his career. Jin and Sun got their escape together.
The point of the sideways was: given everything you want is what you want for you what makes you truly happy in the end?
In the end they all realize that it's not about ME, it's about US. It's about what's best for everyone. What makes me happy right now might not be the best or most moral decision to make. In fact, the moral decision might demand that I sacrifice something.
So, I'm ok with the flash sideways. THAT was the purgatory, if you will. I'm ok with the island being everyone's second chance.
I just hate the almost endless list of loose ends that went nowhere during the journey...