First and foremost, this is absolutely not a spoiler. It is simply a firm statement in one particular direction:
So let's not beat around the bush talking about Sawyer, Kate, Locke, Hurley, Jin, Sun, and I hate to say it, but even Desmond. Clearly the focus of the show will ultimately come down to Jack, his words, his thoughts, and his actions. Of course, everyone else will play an important part. There cannot be a LOST without all of the aforementioned characters. However, I strongy feel that Jack will be the chosen candidate to take over from Jacob. Jack will be in the final scene of Lost. Either his words, or his actions will be the final thing that the audience are left with.
Lost may be a show about destiny, fate or free will, time-travel, the battle between "good" and "evil", and a mysterious island, however, it has deliberately been shown from the perspective of our Losties, and most frequently and importantly, from the perspective of Jack. There is a reason that this story has been revealed to us in this manner, and yes, I partly concede that alot of the plots such as Dharma, MiB/Jacob, The Others, were probably not 100% envisioned in their entireity. In television, writing as you go along does happen. However, so does forsight and planning. So yes, I will concede that some story elements have not been planned, but the reason to show us events from the Losties perspective is because it is the Losties who will ultimately be the main focus of the final scenes, and Losties who will be the main focus of the most important decisions and battles in both universes, and on the island.
Of all our Losties, it is Jack who remains the main focus of the show. He remains the so called "Main Character". Not only were the very first scenes of Lost watching Jack wake up in the jungle, his character development has been like none other. A man of science, preached to by a man of faith that he would one day believe in destiny. He ignores this to escape the island, only for the audience to find out that he does escape the island, and regret his decision. He starts to show signs of faith. He returns to the island. Lockes prophecy is fullfilled, and Jack now preaches about Destiny. He tries to convince others that Jacob has a plan for them all. He lights a stick of dynamite in pure faith that he believes the island will protect him, and it did. He is a man of faith.
Like him or not, Jack is the key to everything.