This is not so much a full-fledged theory as an attempt to rest the case that Sideways World is a fluke and not meant to be.
As I was mulling over Across the Sea, the words spoken by MIB about wanting to channel the light and water to get off the island rang in my mind and the coin finally dropped.
The explosion of the H-bomb released the light of the island, which in contact with the surrounding ocean provided a way for Smokey to be at large and manifest a new world of form from the spring of endless possibility that the source of light represents.
For the light is nothing more than primordial energy, the absolute ground of all being as some spiritualists would have it. But here is the catch, and I am gonna go out on a limb for this part, Smokey represents the mental and most self-destructing aspects of human existence, desire, clinging to the past, the playing out of conditioned patterns. Whether Smokey came about as a result of MIB being chucked into the light or not, the fact remains that MIB's overwhelming desire to leave the island found a channel to express itself through the Smoke Monster. We need only remember how he proceeds by manipulation of the candidates' desire. I think Jacob, on the other hand, represents the Self, the better part of ourselves, the parts that are able to let go, that recognise the ultimate truth of connectness, through the light (Mother's maxim that "A little of this same light is inside every man"). On the interview on NYT, Darlton says redemption through community is one of the most important themes on the show.
And that's why I think the Sideways world will have to go, if there is to be any redemption at the end. The ultimate redemption, that will reward Jacob's faith in humanity, will have to be something drastic, something to "dissolve all illusions", as Desmond already seems to have realised.