The second method will not work because it over-stimulates the emotions rather than appealing to reason. Only reason can produce a long-term change in a person's character. The second method also leads the students to regard moral goodness as an unattainable perfection relevant only to melodrama, and so to scorn the everyday obligations they should be fulfilling as boring and petty.
Kant closes the second Critique with a hopeful note on the future of ethics. The wonders of both the physical and ethical world are always not far for us to find—to feel awe, we have only to look upward to the stars or inward to the moral law in our hearts. The study of the physical world languished for a long time in superstition before the physical sciences began. We can hope that the moral sciences will similarly replace superstition about ethics.
The parallelism between the Doctrine of Method in the first and second Critique is somewhat forced, and we might wonder whether anything is gained from it. The Doctrine of Method and Conclusion are, along with the Preface, the most easily readable parts of the second Critique. Therefore, perhaps the Doctrine of Method not only discusses instruction, but, by closing the book on a strong note, it also serves the purpose of instruction. At any rate, we might wonder whether giving this section the same title as the Doctrine of Method of the first Critique was helpful.
One way in which the parallelism breaks down is that the Doctrine of Method in the first Critique lays out what is meant to be all possible sciences of pure theoretical reason, but all methods of moral education are not laid out in the second Critique's Doctrine of Method. Instead, only one method of moral education is offered. Nor are enough alternatives explored to make any convincing claim, even, that the method here is the best method.
Perhaps the reason that Kant does not discuss other possibilities for moral education is that he does not see any. The only alternative he discusses to his method of case-by-case argumentation is reward and punishment, which is rejected as producing only a simulacrum of moral goodness. Kant pays no attention to the methods that rely on looking at and acting in real-life examples—methods that parents often use long before Kant's method is feasible. So, one might point out features of a situation—"Billy won't share his toys with Susie, and now she's crying. That wasn't nice of him."—instruct—"Be a good girl and give Susie your teddy bear."—and explain—"See how happy Susie looks now? It's good to share your things with others."
Possibly these methods are left out because they do not fall neatly on either side of the divide between leveling brute commands on the one hand and engaging in a reasoned theoretical discussion on the other. One might likewise think that involvement in real-life cases is helpful while learning the meaning of right and wrong, but that once one understands these ideas, purely theoretical argumentation is best for motivating us to care about these ideas. This might be so, but it cannot be assumed.