How Lost will end.
Jacob is coming to the island (himself or Aaron or whoever becomes Jacob anyway).
If Jacob arrives at the island and somebody tells him everything that happened and explains him what he needs to do, he’s going to go turn that donkey wheel and travel back to the past 5000 years ago WITH the smoke monster, keeping him emprisoned for another 5000 years.
– “It always ends the same”
MIB is living that loop multiple times, he’s tired.
– “It only ends once”
Everytime the loop restarts, it’s a new Jacob that comes, so he is not conscious of how many loops have been going on keeping the smoke monster emprisoned on the island. All he knows is what Richard or other people on the island tell him when he arrives on the island in 2010. Actually could be Jacob himself or could be someone else; could be Aaron, whatever. It’s always a new person (or a new version of the same person, for example a new Jacob with no prior knowledge) that’s responsible for keeping the smoke monster on the island.
That also explains why the smoker monster wants to kill anyone that doesn’t leave with him. There has to be no one left on the island when he leaves such that when Jacob arrives, he just lands there and lives alone for the rest of his life, without knowing what to do, or why he’s there.
Now the alternate realities are linked to this theory. The first alternate reality (season 1-5) is the one in which Jacob arrived in 2010 on the island, was explained what to do by Richard or whoever (you have to turn the donkey wheel) and by doing so has brought the island back 5000 years ago in ancient egypt times. For that loop to be maintained where the smoke monster is kept inside the island, we need to be in timeline 1 because we need Jack and everybody to cause all the events that do not lead the island to go underwater. Now the problem is with all the time travelling and specifically the quantum uncertainity on the explosion of the H bomb with Juliet, the first timeline has lead to the creation of a second timeline. The second timeline is as dependent on the first timeline as the ancientegypt-2010 loop is dependent on the first timeline. Something’s got to give.
So it’s either everybody follows the smoke monster/or dies, then the smoke monster escapes, Jacob arrives at the island not knowing what to do, and as a result the ancient-egypt/2010 infinite loops is interrupted, and the 2nd timeline is realised.
Or Jacob arrives and someone is still on the island, telling him to go turn the donkey wheel. Then the smoker monster is transported back 5000 years ago with the new Jacob, and they have another 5000 years to see whether it “always ends the same”.