They got what they wanted most by LLkitty

I think everyone who was brought to the Island by Jacob was given a promise they will get what they really want and will make them happy. Unfortunately, there are strings attached. Like last nights episode when Dogen told Sayid he was promised his son would live, but he could not be with him. Also he must remain on the island to do a new job.

If you look at Rose and Bernard, they wanted to have her cancer go away, The cost was staying on the island. Miles wanted to find out the truth about his father. Daniel wanted to heal his mind and make his mom proud. The price was his life. Sawyer wanted to kill Locke's dad, and that magically happened. The price he has to stay on the island and live with the consequences. Richard Alpert wanted to be free from the boundries of slavery and never age. The price, he is tied to the Island and live forever which could get old after a few hundred years. Claire wanted to give up he! r baby, the price staying on the Island while her son was on the mainland. Michael wanted to be with his son, but the price was too high when he killed two people and had to live with the consequences. John wanted to walk and understand his Dad, the price was to stay on the Island. I think all of the candidates have been to the Island through several lifetimes (reincarnation) or timelines where they battled their demons struggling for the perfect balance of good and evil, yin and yang, whatever you want to call it. It will only end once someone is in perfect harmony and finally achieves the perfect balance in their life. This will be the candidate that will take over for Jacob the body and MIB the soul. I think the reason they were not supposed to leave was it unbalanced the island's energy and the Universe wanted to course correct by making their lives off the Island not quite right. They needed to be back on the Island to finish their lessons with their group of students.! If you notice the alt timeline many things are different such! as Rose acceptin her impending death by cancer, John finally giving up on consultations to fix his back and has accepted his disability. Hurley now has high self-esteem and views himself as lucky. Claire wants to keep her baby (maybe naming him Jacob) etc... In order to meld the people on the island with their alter selves will be accomplished by Desmond who is able to choose what time line he wants, like after he turned the fail-safe key.
Now that Jacobs body is dead everyone is freed from their promises and can leave the Island if they want. That is why MIB told eveyone they were free to go because Jacob is dead. I think the MIB just wants to finally wink out of existance or be able to feel again like the cursed pirates and the Aztec Gold.