It occurs to me there is also a theme of absent/deserting parents; Sam's father leaves him and his mother, of course, and Alex' father leaves in an even more drasting manner. You could argue that for both Sam and Alex, Gene becomes in some ways the replacement parent, and of course in Narnia parents are absent as well, and Aslan is there (but he is not a tame lion...). But Alex, as opposed to Sam, has become a parent herself, and she promised to return; if she were to remain in Narnia, it would continue the series of desertion/betrayal, and in order to break it, she has to return to the real world.