The Great King David fell from his exaltation because he had sinned. No amount of repentance, regardless of how sincere he was, could make up for that fact. He had lost his exaltation. D&C 132:39
If I sin all my life and repent when I'm older, I would have lost out on all that I could have had. Perhaps I might have prophet or saved many souls? The person who sins and repents is not on the same ground as someone who never sins in the first place.