By the end of the series, I see the island and it's magic 'lost'/destroyed/inaccessible to anyone post 2007, Fall of the House of Usher/The Tempest style, so the island sunk fits with that. Surely the bomb's detonation affected the island more than simply sending them back to 2007, but I can definitely see the 'reset' events being caused from something other than the bomb and that the 'reset' events are the end story for most of the characters - but no way Jack, Kate or Locke are living out the rest of their days on the mainland. Their 'LA X' versions will find the island somehow, even if they have to go to the past, with Ellie's and/or Widmore's help 'natch, to get there. But they've already played out that "we have to go back" storyline.
Obviously, Jack's neck cut, Desmond's appearance on the plane and Juliet's "we should have coffee sometime" line to Sawyer were set ups for payoffs & reveals later in the season.
I'm still interested in what/who is Jacob? MiB appears to be a supernatural creature/demon/devil trapped on the island somehow but is Jacob also? We know he can leave the island unlike MiB. He doesn't age, but neither does Richard and Richard is just a man who possibly used the Lazarus Pit water while uninjured (would explain why he's "immortal" and Sayid and Ben, maybe Illana as well, are not). Perhaps Jacob is just a man too, one of the first, if not the very first, to crash on the island (who first used this fountain of youth?), and he entered some sort of contest/bet with this demon. If so, perhaps his "power" is simply foreknowledge of the events that will come to pass on the island he received from some future someone (Jack)?
Someone healed Locke and Rose, but maybe it was MiB/the island and not Jacob's doing. For whatever reasons, the ability of the MiB to leave the island and "go home" seems tied to his killing Jacob, winning the wager and controlling or destroying whoever/whatever Jacob was protecting (the temple & water?). And how could he "go home"? physically via Frozen Donkey Wheel or philosophically through some spiritual release/death? And if he does leave the island and go home, and his power is tied to that of the island itself, could his departure/defeat be the cause of the island sinking? Whatever it is, a free demon/devil/supernatural evil dude is never a good thing for the rest of the world.