3. It's An Historical Documentary
The Theory: So here's an unorthodox twist to take that'd legitimize the events we're watching, but would explain things a little better. So say Kitai and Cypher
areon Earth, and the threat is real. They fight to survive and win, return home, and make a historical record of what they've done in order to chronicle their journey's peril and warn others who may try to make the same journey. Kitai learns through this that his selfishness helps no one, and that to be a Ranger he must not only face mortal peril, he must learn how to help others avoid it.
Supporting Evidence: This might be the weakest theory, in the respect that there's not a lot to support the film will take this route. However, at the same time it's more plausible in the sense that Shyamalan (at least, to my knowing ledge) hasn't tried to pull this one over on us yet. He knows the world is sick of his "twist" film-making reputation that he decides to twist expectations in a more feasible, traditionally Sci Fi way.