It would seem that anyone brought to the island to be a "candidate" needed an inherit quality of wanting "power." The island guardian was all-powerful. A weak candidate would mean a weak guardian - - - the island would be in jeopardy. That is true if the island was a fragile object that needed human protection.
The final candidates did not have the same power traits. They may have had common personalty features like free will and primordial need for survival. We can group the final candidates in a simple chart:
The one person of this group who really wanted to be The Leader was Locke. But he had to leave the island in order to follow his destiny as told to him by Christian (or MIB). Locke was manipulated into thinking he was a final choice for guardian because he thought he had a spiritual connection with it. Locke was manipulated into leaving the island instead of guarding it. Locke could never have succeeded Jacob because Locke was too naive and lacked judgment. Locke's purpose was to die so that more worthy candidates could return to the island.
Sayid and Jin never expressed any desire to rule the island. Both were focused in on leaving the island; rescue. Sayid left the island, Jin did not. Sayid's post-island life was also a manipulation (by Ben). But Sayid's return was not one of leadership but one of revenge. He was to be a tool or weapon in the final fight against MIB.
Sawyer never wanted responsibility. All he wanted to do was get off the island. Anything that stood in the way of his goal was pushed aside. Sawyer was selfish. He was never comfortable in any leadership role, even when Hurley pressed him to be the "de facto" leader of the beach camp. He never wanted to attain a higher calling in life. Like Sayid, his life's motivation was consumed by revenge.
So in all of the convoluted story arcs and twists, there were only two viable final candidates from the very beginning of the series: Jack and Hurley. Jack accepted the notion of leadership because the survivors immediately valued his medical skills as a means to their rescue and survival. Hurley never accepted a leadership role. He was comfortable at the end of the mission line. He hardly made his opinions known until the last story arc. He did not have the ego to want to seize power over other people's lives (because of his feelings of what other people in a similar position did to him).
In the end, both Jack and Hurley become the island guardians. One volunteers and one assumes the job involuntarily. Jack's reign as guardian is short lived; Hurley's reign is one of janitor, cleaning up the leftover strings, to close the island down (whatever that meant). We can assume that Hurley sealed away the island from future "crashes," but that is one of the final mysteries: what ever happened to the island after Hurley?
Nothing. Hurley and Ben briefly discuss their time on the island in the sideways church courtyard. There is no great revelation that the island needed protection, that it had a new guardian or that it even survived at all.
The island either continued on its life force mission without a guardian or it faded away from reality when none of the characters remained on (or in) it. In either event, the world did not end. The only thing that ended were the lives of the main characters who gathered in the after life church to move on.
