Island would’ve sunk/did sink if/because the plane never crashed
The sunken island is what would have happened if the plane had never crashed and had landed safely in Los Angeles (flash sideways of the fate of the island). I believe this is because the candidates never would have landed on the island, leaving Jacob’s position wide open after he is killed, allowing MIB to leave the island and releasing evil into the world, forever rendering the cork (the island) out of commission.
This just came to me, and I understand there are probably a lot of details that don’t fit with it, so what are your thoughts?

but if they didnt land, would jacob have died? the people on the plane were necessary to MIB’s plan
Not to mention if the island sank because the plane didnt crash, then why would it already be sunk as the plane was in the air. The effect would be that immediate? I dont think so. Not to mention Jacob was killed in 2007, 3 years after 815 landed. My guess is the island was sunk to prevent Smokey from escaping because the candidates on 815 werent ever part of the equation in the world of the FSW. Or maybe in that reality Smokey sunk the island somehow to prevent its use again, and he has already escaped.
“If the plane never crashed” Ben never stabbed Jacob, and he’s either still around or he AND MIB drowned.
“If the plane never crashed” is what the expanded always refers to Sidewaysville.
Therefore, ‘if the plane never crashed’ the island sunk, sometime after Dharma came.
If the island sunk before Sawyer’s parents died, Jacob wouldn’t have touched any of the candidates. (Again)s he and MIB may have drowned … leaving no evil to escape (Sidewaysville doesn’t seem ‘extra evil’).
I’ve written a few theories on this, pointing out that the life shown continuing in Sidewaysville is the life the losties had before they got on the plane (because it had to crash for what we’ve watched over the last 5/6 seasons).
That means Jack had a son named David, Ben left Dharma and became a teacher, Miles and Sawyer became cops, etc.
And, it *could* mean that the spread of evil was naturally stopped, ‘if the plane never crashed’ … logically follows the spread of evil was not (yet) stopped – ‘if the plane crashed’.
(BTW: Jughead probably had *something* to do with it.)