"only ends once" means the end of mankind. This is what the Valenzetti equation is all about....what the numbers are all about. Remember the goal of the Hanso foundation: to change the numbers so that the end of man is delayed or wiped out. The Valenzetti equation reveals the date upon which then end of mankind is to come. This date is encomassed in the numbers. Those numbers MUST be changed, or the world comes to an end.
I believe the sideways flash is leading up to that end and the Losties on the island will meet the destiny of what occurred in season 5 with the blast. That was destiny (remember the ads). Our Losties on the island must change something so that the flash sideways does not occur. The plane onlyh crashed because of Desmond. Should not have occurred. Without that event, then the plance lands, the Losties move forward and the Valenzettiu equation happens. It is the island and the times shifts that occurred and everything that the hanso foundation has been doing to change all of that to save the world.