I'll say it again. Daniel said it himself. Desmond is special, he can change the future, no one else can. He is the ONLY character that has actually changed anything. Everything else already happened.
The story isn't about how it will end (meaning the purge), it's about how we get to that point. I don't think the last episode will be the first one, I think we'll move beyond that somehow. But the story right now is explaining the island's past--Dharma's past, Ben's past. it happens to be the Losties present, but it's the writers way of explaining the past without flashbacks so that we can move on to the future understanding the past.
We know there's a purge--but that is all we know. We don't know why, we don't understand the relationship with Dharma and the Hostiles, we don't know Ben's motivation. We're in the past to learn all of that.
When this is all over--you guys owe me a cookie if I'm right.
And remember my theory (and Daniel's since it's the same) leaves room for varibility because of Desmond--it's the writer's tool to fix any inconsistances.