My theory is that very few people are actually trying to warn children about the dangers of believing in Magic; they're just writing standard Resign Yourself To the Bleakness of Life in General stories that use magic as a hook (because otherwise they would just be standard Resign Yourself To the Bleakness of Life in General stories).

That, or magic is being used to represent art and creativity and imagination, so the message isn't really "magic isn't real" but "it's time to put away art and creativity and imagination and all other childish things and go work at the box factory." I don't entirely buy that as a theory, but it would explain why they haven't been getting published since 1996 - no one works at the box factory anymore, and children might need creativity and imagination to succeed in the knowledge economy.