So let's re-cap all the hypotheses here and the major problems with each of them.
1. She was murdered by someone in her family.
Not killed, but
murdered. I have seen a few people suggest this: Melissa was sooo angry that she just strangled her to death to avoid having to deal with her and her pot-smoking shenanigans. Needless to say, this is more than a little far-fetched.
2. She overdosed, and her parents covered it up.
This appears to be a very common explanation of her disappearance. To avoid endless delineations and arguments, I will simply cite this article, which I believe to be very thorough:
No Voice, No Purpose: The Lies and Fall of the Karlie Guse Lynch Mob Though I disagree with some of the emotional pleading ("You can never blame the family members!"), it's well-argued and deserves to be looked at by anyone assured of her parents' guilt. To be clear, I think a lot of the parents' behavior is unusual, but I also think they deserve a great deal of sympathy and compassion. The risks either way, that we ignore possible suspects or that we blame innocent victims, can not be overlooked.
3. She was kidnapped by BM's boyfriend
This is one some of you may not have heard, but I was told by the person who wrote the article above (Damion) that the police were considering this angle. Supposedly, he was driving through the area at the exact time Karlie disappeared. It would certainly explain some of BM's odd behavior. I can't give away any more information.
4. She wandered out into the desert and died.
Before learning of the extensive search effort that went into Karlie's case (it really was and still is a cause célèbre), I thought this was the most likely explanation. Now I do not consider it so likely, because even though she was disoriented and seen entering the desert, she could not possibly have gotten very far in her condition. She did not drink any water before leaving, nor did she take any with her. She would have succumbed to the desert heat within a day. How far can a disoriented girl trek into the desert? That's how far and how long it would take authorities to find a body. Consider this explanation bunk.
5. She was kidnapped by an unknown assailant.
Most likely. This is the only explanation consistent with everything we know.
6. She's OK, just hiding
It's happened before. But she didn't take her glasses, and she didn't take anything else. She didn't go to any of her friends houses. Is she a desert nomad now? The vast majority of missing persons return before 1 year; only a minority of missing persons return after 2 years. And beyond that point? You can consider the missing person dead. We've all heard stories of people who "went missing" and changed their identities, evading authorities and everyone else. The woman who imitated Brooke Henson being a prime example. However, how common is this? Considering Karlie's circumstances, this should not be given precedence over the kidnapping explanation.