Back to the Future, of course, is the fantasy: in it, Marty destroys the circumstances of his own parents' meeting, and in restoring that, actually comes out with "improved" parents, whose character and quality of life are arguably much greater than they were at the beginning of the film. Cast Away, while dealing with an extraordinary circumstance, in fact is ruthlessly real in its conclusion, where Tom Hank's Chuck Noland speaks of his coming to one final realization: that the ultimate duty of human life is to keep breathing. This is not to say that to "keep breathing" is the sum purpose of human life. This is our duty because despite the clear and brutal logic that our time of despair always presents to us, we are inevitably graced with possibilities that were unforeseeable. In both films, the protagonist loses the life they had known to that point, trading it for an unknown future. Occasionally this happens in life by choice, but more often by circumstance. In that, perhaps, the movies actually do reflect real life rather accurately. We might also find that our choices are often really "circumstances" in this sense, in that very rarely do we make our life choices with a strong or complete sense of their consequences.
It is the fantasy story that is consistently replacing defeat with victory, and turning setback into comedy and delight. Marty McFly moves from the life he had known to a better life, and is given the miraculous opportunity to "correct mistakes." What could be more fantastical? Chuck Noland, on the other hand, has no such opportunities. After the years it takes him to return to the life he knew, he discovers that that life no longer exists: his beloved is married to someone else, the mother of someone else's child. He has been brought back to life, but not the life he wanted or expected. This is our natural condition, I suspect, but one that we tend to hide from ourselves as much as possible. Despite the seeming-control of our decisions, we can look back at our lives, particularly at the intersections and crossroads presented to us by other people, friends and lovers, and ask the haunting question of "What if?"
Is it a worthwhile question? Can we gain wisdom from it, perhaps leading to better decisions at future crossroads? Or is it a distraction? A chasing after fantasies, illusions we conjure up of "might-have-beens" that have little possibility of having come to pass as we imagine them? After all, lacking that flying Delorean time-machine, we do not have the opportunity to correct the "mistakes" of our lives, to flip the pages back and choose a path we had not taken previously. The best miracle we are given is to accommodate ourselves to past choices, to heal old wounds, and to hope for new possibilities that former ones would have precluded, yet with no certainty of reward, and no comfort of control.