Here are some more thoughts about LOST. Maybe Jacob was wrong when he believed that the light on the island was necessary to sustain the earth and the people on it. He was wrong about other things. At one point, he believed his mother, who said that the people on the island were the only people in existence. But he changed his mind on that, when he learned differently.
Perhaps Jacob was like Desmond, who, in season 2, continually typed in those numbers, believing that he was saving the world. But, if I'm not mistaken, Desmond was not actually saving the world. He was trying to contain the island's electromagnetic properties. When he failed to type in the numbers, the entire world did not collapse, but electromagnetic energy was released, which brought down the plane.
It's interesting that, in the parallel universe, the island was sunk, and yet life went on just fine. I'm beginning to doubt that the "flash sideways" itself was a timeless universe. Rather, I think it was a stepping stone that God used to get the island survivors to that timeless universe, which included the church at the end of the episode.
What about some of the mysteries of the island, such as the fact that many of the women could not have children there? There are different possible explanations for that. Maybe it had to do with the island's properties. A blurb on the Pilot said that Locke and Rose were cured by the island's electromagnetism. Perhaps that electromagnetism kept women from having children. But what about the women who did successfully have children on the island? Well, there aren't necessarily absolute rules. Locke and Rose were healed by the island's properties, but Ben Linus was not---when it came to his brain tumor.
Or perhaps women couldn't have children on the island due to the guardian, or the smoke monster. The mother of Jacob and the Man in Black didn't like humans on the island, so she could have limited their ability to reproduce there. Or the Man in Black didn't want there to be more possible candidates to take Jacob's place.
I thought the finale would give more attention to the question of whether or not humans are inherently good or evil. But that was Jacob's baby---it was his project to prove that humans could progress, against the smoke monster, who believed them to be corrupt. That's why Jacob brought people to the island. One did not have to have that project to be the guardian of the island, since Jacob's mother did not want humans there, and viewed them as corrupt. But it was Jacob's project, and it was significant in that it provided a context for the characters of LOST to grow, which was what was important.
And yet, to contradict myself, didn't the island bring people to it, since there were people there even before Jacob was born, grew up, and started bringing people to the island himself?
So we have a weird island. We have Jacob, who may think his mission is greater than it really is. And we have his little debate with the Man in Black about the nature of humanity, which God still uses to build character in the island's inhabitants.
The light. I'm not sure entirely what that is. People come to the island to use it, apparently. I think that is why there is interest in the island, and why Widmore and Ben were continually fighting for control. Maybe one of them, or both of them, wanted to exploit the light for some purpose. But the light was necessary for the island to survive, so perhaps the island itself saw its need for a protector, and the island somehow conveyed to the protector that the existence of the entire universe depended on that light (which was an exaggeration). The protector was conned, as Desmond was, when he pushed those buttons, thinking he was saving the world. And the departed souls on the island (i.e., Richard's wife) also believed that the fate of the universe depended on the light. But, as I said, the parallel universe did perfectly well without the light, or the island.
One more point: I like something James McGrath said in the comments section under "LOST Places In The Heart: Making sense of LOST now that it is over (SPOILERS!)": maybe Ben didn't go into the church because he hadn't yet formed meaningful relationships, and he wanted to do so in the parallel universe. Ben felt that he wouldn't fit in, yet there was still time and space for his redemption.