I have a theory/idea for how the FS could become relevant somewhere down the line.
What if the FS timeline is all that really exists anymore, and the only remnants of the original timeline that exist are simply the island and the people on it? As Desmond said when he returned to the island on his boat at the end of series 2, they're "trapped in a bloody snowglobe". At the time he said this, it turned out Desmond was mistaken, but what if Jughead/The Incident caused such a release of energy that it destroyed the outside world, and they are now trapped inside a snowglobe? And the only thing that exists is the island, unstuck in time, skipping around time and space, while the rest of the world is destroyed.
Of course, this begs alot of questions. Firstly, if my theory is true, then howcome Widmore still made it to the island? Well, remember when it was said that if you were in a certain proximity to the island, then you move with it, well I propose that Widmore has purposely stayed within this boundary to make sure he moves with the island. He then tactically approached the island when he thought the time was right.
Widmore has foresight that the whole world will be destroyed, and that when that happens, only the island will be skipping around time and space. So Widmore made sure he got himself and an army in proximity to the island, firslty, to save his own life, and secondly, to save the only thing he cares about: Penny. Locked up in the secret Sub door will Penny, along with Desmond and Charlie, which will further indicate Widmore's redemption and good nature. Not only this, but Desmond will have a key role, as he is "uniquely and miraculously special".
The Second question would be "If this theory is true, then how will MiB still make it home, if home doesn't exist and only the island is left?". Well the answer is, he can't make it home. Only the island exists. Widmore knows that once MiB finds this out, he is going to go crazy, and so Widmore and his army are attempting to subdue and capture smokey before smokey can find out that only the island exists.
The Third question this theory raises, is that "isn't it a bit much of a stretch? Only the island exists and the outside world is gone?" Well on one hand it is a stretch, yes, but on the other hand, it would perfectly explain the reason why so many random people are invested in island-events. For example, it would explain why people like Eloise are so concerned with on-island matters, and consider them to be of such high importance for all of humanity. If it was just a case of who rules the island for the next 200 years, would Eloise really still be so heavily invested in matters that she needs to be a temporal policeman, and guide Desmond safely to his destiny? I doubt it. I think she is only so heavily invested because she considers the outcome of these events vital for all of humanities survival.
So if the outside world is gone - is there any hope? Well, I believe strongly in the prophecy of "What lies in the shadow of the statue? - He who will save us all". I believe whatever it is that lies in the shadow of the statue will be the one to save us all. I.E. He is the only one who can save the world and the island.
Think about it! In original timeline, the outside world is destroyed, and only the island exists in a snowglobe. In FS timeline, the outside world exists, but the island is destroyed. Perfect opposites.. a perfect balance. This is not a story about MiB and Jacobs rivalry, this is about the bigger picture. This is about balance on an altogether different plain.
Now and again I like to propose theories, not necessarily because I believe they are 100% correct, but because they are thought provoking whether they turn out to be true or not. I believe this theory has some merits, however I realise it is also highly speculative. Speculative or not, I hope it creates some discussion...
1. Juliet blows up the bomb
2. The island breaks apart, people escape, it goes under
3. The world goes on, but it is a world that carries on from 1977
4. Events are different from 1977 onwards, hence the flash sideways we see.
5. In 2007, the losties from 77 flash to 07, renewing the original island's existence
6. There is no Original outside world to speak of, that carries on. The world that carries on going is the FS timeline - it has become the only timeline. The outside world is actually now the FS timeline we are watching.
Now this theory, coupled with my theory that Juliet's consciousness is floating between both realities in LA X, I believe provide a rounded theory as for what will happen in Season 6.