So, you think we're seeing "what happens after the end of the story", before the story ends?
I've been starting to think that as well. Telling two stories is what they've always done. The entire show's "Flash" phenomenon has been used to develop our characters. What better way to complete their story arcs than to finish them that way. Treating them not just as the characters who have been lost for six seasons, but rather as the characters who came to the island lost. Why they were lost, took us six seasons to discover, & it all started with specific events, in their history, i.e. Sawyer's parents dying. This is why the lighthouse mirror shows those visions
These events were a turning point, where their lives went down a path toward them being the lost people we met in season one. These new realities, are the people they should have been, were they to not let these life altering events effect them the same way. They are now the people who have learned the necessary lesson, that we've been watching them learn over the whole series, on the island
I'm not entirely certain that this suggests the island is purgatory, or a guardian angel, like in "It's A Wonderful Life" but it just seems that this time on the island was there for them to learn who they are & should be, & this alternate reality is them now living with that understanding
This will have to make sense in the island reality too, because the island is also a character, otherwise, why show us all the Dharma stuff, & other island history. I'm certain, by the end, they will have given us a better understanding of what the island is, or represents, such that these people's lives have had this development there. Will they answer all our questions? No, but I think we'll be able to piece together many answers we seek, once we start connecting the dots, by examining the entire series, as a whole
We'll be pondering this story for some while, after the finale airs, without a doubt.