My guess is that there is only one full-scale reality, which contains a separate mini-universe (the Island). The timelines of each is mostly shielded from the other. So, changes in one timeline won't necessarily affect the other, except in extreme cases like exploding the nuke in the pocket of energy.
So, what we're calling the flash-sideways reality is the only full-scale reality. It is the main timeline. It was just modified by the events on the island. So, there's no way to go back to the 815 crashes reality. That has been changed to the FS reality.
The big question is, do the islanders stay on the island, or do they go to the mainland, which would require bringing MiB with them. Presumably MiB will make returning to reality very attractive to them personally. So, it'll become a question of personal benefit versus global costs of having a manipulative smoke monster running around.
Enter the reason why the writers have taken so much effort to make us care about the new reality. The islanders will have to forgo what their mainland counterparts have acquired. Which in some cases, (Hugo with Libby, Jack with his son) would be difficult. But, other cases not so much (Sayid), although even he would know that Nadya is alive.