My theory is they meant the lesson to be learned by santa and the others, through Rudolph, but they focus on him so much that you expect Rudolph to have learned or grown himself. Truth is its santa and the head elf who grow, by realizing that even outcasts have something to offer, and that being cruel is, while morally wrong, just dumb as shit because you might need a glowy reindeer snoot someday, but only if you didn't drive them to run away and die to a snow monster. Or something.
That or they just didn't think it through very well at all. Which is probably more likely. Idk. But it feels a little bit more understandable if you think of Santa as the one with the arc, not rudolph.