It first needs to be stated that in Jacob's search for his replacement, he is looking for more than someone who will protect the island; he is looking for someone to prove the MiB wrong about humanity.
The role of Jacob on the island is the Protector. He accomplishes this role by controlling the MiB's power by keeping him on the island. However, at the same time Jacob is also recruiting or bringing people to the island in an attempt to change the MiB's view that evil is inherent to humanity. This is where Jacob's search for the candidate takes a turn and he begins to focus on finding a candidate who will protect the island by changing the intrinsic nature of the MiB.
Jack is our man for the job. Now that he is the last candidate with the MiB, he will unintentionally through his actions change the MiB's dim view of humanity. The MiB has come into contact with Jack since the third day on the island, as it was just revealed that the MiB took the form of Jack's father Christian to lead the 815ers to water. The MiB and Jack have a past and the MiB knows a great deal about Jack; what makes him tick, the struggles he's faced on the island, and his reluctant role as a leader of the crash survivors. The MiB believes he has Jack all figured out, just like the rest of humanity...inherently evil. But what the MiB doesn't realize is how much John Locke affected Jack. Jack has become a completely different person since leaving then returning to the island. Jack now shares in John Locke's belief about the greatness of the island, an inherent shift of Jack's original core belief system as the Man of Science.
The change in Jack that might sway the view of the MiB could be Jack's continued attempts to stay on the island because he now believes it is his destiny; he felt a piece of him missing without the island. The MiB continues to insult John Locke by calling him a sucker and saying that his belief that he was destined to be on the island is what got him killed. By saying this to Jack, the MiB is trying to plant seeds of doubt in Jack about his own destiny.
The power of the MiB represents a corrupting darkness, apt to spread; much like doubt. The MiB is attempting to use his power on Jack in the form of doubt about his own destiny to once again prove Jacob wrong, that humanity is inherently evil and Jack will once again save himself versus following his believed destiny. However, this is not what will happen. Jack will continue on his current path of following his destiny and continue to make sacrifices for the sake of the island. Jack’s actions will represent something in humanity that the MiB has never seen before and will cause a shift in the MiB’s view of humanity which will lead to a shift of the MiB’s power from corrupting darkness to inspiring light/hope. Once the MiB’s power is neutralized, he no longer poses a threat to humanity. The Protector of the island (previously Jacob and the candidates) and the island will no longer be needed to keep the MiB captive. The candidates lives will change to their original destiny before Jacob interfered and the island will sink to the bottom of the ocean…giving us the Flash-sideways Timeline.