Food for thought - the alternate time-line isn't 'What if Flight 815 never crashed', but more 'What if Jacob hadn't interfered'.
If what Smokey Locke says is true, and Jacob had been there pushing them to the island all along, without that interference their lives could have gone in completely different directions. The most obvious example of this is Locke. Just after the ground broke his fall, Jacob turned up to tell him everything was going to be ok. Subconsciously this may have given Locke the faith he has so fervently stuck with throughout the series, and without it, he turned into a miserable wreck, bottling up his disappointment with how his life turned out. Anyway, I'm rambling and you get the idea. This is my favourite theory so far, especially since the whole rollercoaster time-line the island rides helps explains most counterpoints.
Thoughts?