In short, all of the characters had experienced pain and misery on the island, and they didn't want anyone else to experience the island. And Ben didn't have to spend all that time on the island, and got to, for the most part, grow up in the real world. Sun and Jin had a better relationship, even though they weren't married. Driveshaft was still together, so Charlie was happy. Desmond was finally accepted as a good man by Widmore. Regardless of whether he had a son in real life, in the FSU, having a son was a way of reconciling with his father issues. Jack moved past his father issues by resolving issues with a son. Locke stayed together with Helen, and he moved past his father and walked again. In the FSU, most of the characters' lives were better. The FSU was purgatory, and as Christian said, it was a place created by the characters to meet each other so they could move on. It doesn't really matter physically why the island sunk. If Benjamin hadn't been in the fake universe at all it may have been that the Island wasn't even there to begin with, due to none of the other characters having any knowledge of the Island before the crash (other than Desmond, although he didn't know about it due to Benjamin being there.) Because Benjamin's history in the fake universe still included his life on the island as a child, there needed to be a reason for him to be off of the island and back in the real world, hence the island being sunk underwater. Because of this they all had to be in a certain situation in their lives where they could do these things that satisfied their need for redemption. This world also seemed to be a world where every flawed character could redeem themselves for the things they have done wrong or the aspects of their life that made them flawed like Jack's father/son issues, Ben's relationship with his daughter, Locke's denial, etc. Because of this the world that was created was based on the knowledge of the main characters and the main characters only. We know the sideways universe wasn't real and was just a means for all the characters to see each other once more before moving on. If people have in fact discussed this I apologize for bringing it up, I just have not seen it mentioned and it has been burning me up. I guess by the time they got to the finale they just changed their mind about what the ending should be? Or, they just wanted to fool us into thinking we knew what was going to happen once Locke started plotting to sink the island? While I did like being misled in one sense, it just seems a bit silly to me for them to show that at all. Now, WHY would the directors/producers/etc decide to emphasize the island being COMPLETELY submerged underwater with dramatic music and such and then never show how that happened? Maybe the island ended up sinking long after the events in the finale, but wouldn't you think if they opened the final season with that image they would close the season with an explanation for it? I guess with LOST you would think a lot of things though, haha. Jack ends up saving the island, which Hurley and Ben protect for, presumably, years to come. In the finale, Locke reveals his intentions to sink the island, but that is not how it ends. This is what has been driving me nuts the most and I haven't seen anybody talking about it, so maybe I am missing something.