Saving the World, Part 2 by Jhisc9999

My previous theory was as follows:

"How is pushing a button saving the world? Let’s consider what we know. The button releases a controlled amount of energy that builds up as a result of the incident. What also has existed since the incident? The ALT… I believe that the Swan has been maintaining the existence of the ALT ever since it was created. Just saving the world. What world? The ALT world.

The universe does not like an alternate timeline and has been trying to build up enough energy to destroy it. But the hatch and the button prevented that, until around the end of 2004 when Desmond turned the key. That’s when the ALT ended. For some reason, there is a 3-year time shift in the connection between the timelines. So we are seeing the ALT before it is destroyed. But time is running out.

The ALT has some purpose that it needs to fulfill before it ends"

To continue, I think Widmore and Eloise are working together to save the ALT. They have sacificed the people they love in the original timeline as a means to this end. Eloise told Desmond that pushing that button was the most important thing he will ever do. But her knowledge only went so far. Widmore has brought him back to the island to undo something else that he did.

I believe he wants Desmond to enter "the source", the cavern of light. But instead of being turned into a smokey, he will be able to withstand to forces and will end up back where he started just before he turned the failsafe key. This time, though, he must not turn the key. Turning the key destroys the ALT, not turning it, destroys the original timeline.

I don't know how Widmore's plan will be received by the other Losties. They may try to stop him or they may try to help him. I think the latter once they realize, through Desmond's help, that there is an ALT and that everyone is still alive in it. Even Jack will sacrifice being "the one" and will go back to being the hero trying to save everyone by finishing what he started with his Atom Bomb plan.