Maybe better understanding of what the island & the loop actually is will help to get the "big picture" of LOST.
The simplest explanation of what the purpose of the island is: life. The island is perpetuating life as we know it. Here is a simple timeline of the events in the loop that we know of:
The Hatch is built. Des doesn't enter the numbers. Flight 815 crashes. Failsafe is turned. Hatch implodes. Turn FDW. Losties travel back to 1977. The Incident occurs. The Hatch is built. Everything happens all over again. Again and again. Hence, the loop.
This is based on the notion of the loop (not on whether the losties did or did not stop the incident from happening). I'm just talking about the purpose of the island and the loop.
When Des turned the fail safe (breaking the loop), he created the alternate reality from the release of the EM radiation. In this new reality, life continues to go on normally everywhere else in the world (obviously except for the Losties). This timeline branches off from the exact moment when the failsafe key is turned. While everything is peachy in the ALT reality; in the original reality, the loop continues to happen again and again.
Everyone needs to know the importance of this loop. The Loop perpetuates life. It does so by creating ALT dimensions/realities. The Island is prolonging life as long as the loop is looping. Each time the fail safe key is turned, a new dimension is created at that moment. That new dimension's timeline continues to survive until it's end. It could end by something catastrophic, or something entirely different. But the important thing to realize is that while the loop is looping, another timeline with be created the moment the failsafe key is turned again. Therefore life continues, and it's survival is driven by the loop. You can view this as sort of a "backup" for the human race. If something happens to one timeline, the loop will create another.
Much like Stephen Hawking's belief that we must colonize more than one planet incase something happens and we go extinct. You can't put all your eggs in one basket. The statue of fertility could be reminiscent of this idea, after all, it's on the island.
This is why the loop is so important. The loop ensures life continues to be lived, time continues to tick. The Valenzetti Equation anyone? Since the numbers are based on the calculation to the extinction of the human race, I think it fits.
"It only ends once." = It takes one element to destroy everything.
"Everything else is just progress." - The reason Jacob brings people to the island is to perpetuate the loop. It's progress by keeping the human race alive & kicking by creating off-shoot realities.