It only ends once... And it's going be Desmond who ends it by KING_CHAN

Perhaps this theory is already out there and should have been coming earlier, but with the 3 years difference between the Original TimeLine and Alternate TimeLine, I had trouble wrapping my head around this idea at first.

Each time Desmond experienced a flash it would take him to a time that's completely random. In "Flashes Before Your Eyes", after turning the failsafe in the hatch we see Desmond back in his apartment with Penny in the OTL but with certain memories of his future on the island. Whilst in "The Constant" we see him back in constantly drift between his army days and the present whilst travelling amongst the perimeters of the island, and eventually have his memories intact after finding his constant - Penny. And after "Happily Ever After" we see him flashing to some time after he got off Oceanic 815 in the ATL, a time that's different to Jack's initial flash whilst on the plane.

And now we see the light in the center of the island, Desmond goes in there and the light is going to try and take his soul, but because of his special ability to withstand electromagnetism the light would get bigger and bigger in trying to kill him, hence we have recreated the burst of EM that we experienced at the end of Season 2, but this time there's no more failsafe. The light engulfs the entire island, the world, eventually replacing it with the ATL, everybody on-island (alive or dead, including Dogan, Mother/Jacob/MIB) will be there, the light goes out and the island sinks with it...

I don't know about you but I want to see what happens when the light goes out, and that MIB gets his wish to see what's across the sea but only as an ordinary human being...

Now the writers have left what Mother knows of the light ambiguous (like they always do) and we could all speculate she doesn't know exactly what would happen if the light goes out. And she didn't know someone like Desmond with his special ability and thinks anyone who goes into the light would just turn up as smoky. Same with Dogan, they think death on island would simply relive them of their duties, the thing they didn't know is that they get redeemed for their sins by their deeds on-island in the flash sideways, just as for all our Losties.

Now we could argue (almost obvious) idea that Jack is the replacement for Jacob, what's his role in protecting the island? But we've known Desmond is special for a long time now and his specialty is what cause the island history to end.