Its just progress..
I got this from the first 5 minutes of the finale in the conversation between Jacob and the MIB
The MIB says at the start “They come, fight, they destroy, they corrupt” > This could be linked to the Losties time on the island.
I think the Losties might not get sent back. Jacob’s comment could also be a hint to this > “It only ends once, and what happens in between, its just progress” So that the explosion or flash wasnt the end, it was just another point in the timeline of the Losties, leading up to the end. And whatever happens after this, is also progress.
“Whatever happens in between, its just progress” This could also be linked to course correction. Perhaps the Losties are on a timeline without knowing it, and the ending that is coming up was always going to happen since Episode 1, Season 1.
So what this is saying for next season, is that the Losties could be sent back to Day 1, or to normal time, and all that is just the next point in the timeline.