Everything That Has Happened Happened
I’ve been thinking of the flash sideways like in the book The Dark Tower. The Incident caused a split in time and in the minds of the Losties. They are living 2 lives, on the Island and in the flash sideways. Their minds split in 1977 and they all have a weird sensation that there is some other memories but they can’t recall them (like an itch you can’t reach).
The Incident caused the plane not to crash which caused them to not be in 1977 to prevent the energy from Radzinky’s hole from sinking the Island and in the flash sideways the Island sunk. But the Island also did not sink because they were there to drop the bomb in the hole so there is also a reality where it did not sink. At some point they will do something that will merge these memories (or Jacob planned for the natural course correction we have seen). The questions I have are: will the Island time become the main reality or will the flash sideways become the main reality? Will they all need to find Constants like Desmond had to with Penny? Will they all be each others constants?
My theory is that everything we have been shown has happened (and is happening) and the bittersweet ending we have been told about will be Jack (or someone) staying behind on the Island to keep Flocke trapped while everyone else merges. Perhaps the merger happens with death, this is why Widmore says he did not kill the Ajira passengers (but he did not expect Sawyer to believe that). Jacob and MIB know that they are trapped between realities and this is where the prison is for the Evil we are seeing. I wonder if we will find out how this “in between” was created as a prison for evil? That would be cool.