Here's my theory on LOST.  The alternate timeline is what's going to happen if they can't stop the Man in Black from leaving the island.  If the MiB leaves, it'll kill the island destroy everything the Losties ever did, and we'll only see what their lives will be in alternate timeline.  If Jacob wins the "battle", then MiB will continue to be contained on the island by Jacob's replacement, Jack and so the battle continues.  Jack and Locke have never seen eye to eye so having them continue what Jacob and MiB started would be a cool way to end the show.