I believe the sideways flashes, or alt universe, as many are calling it, is EXACTLY what happens. Although throughout the seasons we have seen flash backs and flash forwards, we have never seen any part of the story which was not to be taken as factual.
Just as many viewers were confused after seeing the two minute section of the final season's opener, I too thought "What the heck is going on? Is this what would have happened? Is this a previous plane flight where they were all present?” But as the camera panned out of the plane’s cabin and into the ocean where the island was leagues below water, it occurred to me that this IS the season's finale being unveiled slowly through each episode.
The creators of the show have always said they knew EXACTLY how the show was going to end since the fist episode. They also never planned on so many seasons and/or characters.
The creators knew how the show would end. They knew this before Ben Linus, Daniel Faraday, Jacob, Mib. This leaves me to believe all of these characters and all of the events after Season 1 are extraneous and unnecessary to finish the story.
The final episode of LOST will see one of the candidates moving the island below water (potentially forever)by turning one of the gear wheels located underground in one of the Dharma stations. This will create a time continuum paradox that would have even Doc Brown confused.
With the island moved below the sea, there would be no more inhabitants, no Others, no beach, and no crashing of Oceanic 815. While sitting on the island, Jacob and MIB talked about the rules and loopholes. This very well may be the loophole of which they were speaking. If the passengers never crashed on the island, none of their activities on the island would ever have happened. This includes what happened when they initially landed on the beach as well as all of the mischief they caused when the island was jumping through time.
This is just one of many similar examples:
Remember, without the crash, Daniel Faraday would never have gone to the island with his journal. Locke would never have moved the island. The island wouldn't have been skipping through time. Daniel never would have come face to face with his mother in the past. Therefore, his mother wouldn't have been able to kill Daniel and give him the journal in the future.
In the finale we will see the resetting of the island and we will realize the nature of the 'alternate' universe. The second to last scene in the season will show one of the candidates 'moving' the island under the sea. The final scene of the series will be the EXACT scene that opened this season.
Turbulences...Jack...Rose etc. This is when the audience will understand what they have been watching in the alt universe. They have been watching EXACTLY what happens when the plane DOES land.
Through each "flash sideways" each time one of the castaways from Oceanic 815 comes into contact with each other and/or previous residents of the island they each have a double take or pause. The same phenomenon cannot be said of the Others when they see Jack, Locke, Kate, etc. This is because, they have never resided on the island. Ethan, Ben, Dogen etc.
The big question is why. Why did they crash on the island? Why go through all of this? Each character goes through his or her own trial and each comes out a better person because of it. What I mean is, when they get on the plane to take off from Australia, they are each the damaged soul we have learned to love, but when the plane lands in LAX, each has grown, or been fixed to be the person we see in the sideways flashes.
Jack is not an alcoholic.
Rose is not afraid of her disease.
Hurly is the luckiest person in the world.
Claire has Aaron and raises him.
Without Oceanic 815 crashing many of the events that we have seen in the flashbacks will not have happened either. Each week we will understand more how Jack has a child and is married (to Juliet I bet), Kate is known by an alias, and why Hurly is lucky. We will also learn why these discrepancies exist. I understand this theory doesn't explain Smokie, Jacob, Christian, the numbers, nor who lives in the cabin, but the simplest answer to a question is often the correct answer.