The question is not clear. Has a comment to be made on the way the poem ends? Or,an alternative end to the poem is to be suggested?
Anyway, looking at the first option, the end is absolutely justified. The nightingale received what she truly deserved. She was out and out a brainless bird. She could not see through the game-plan of the frog who wanted to destroy the throat of the bird or her life. Ambition made her vulnerable to exploitation. Had she been intelligent, she could simply understand that he was overtaxing her and making best of both the worlds.
If the frog were so talented and Mozart in disguise, he could make money by singing himself. However, he trapped the poor bird in his malicious game and got her killed.
The second option stands null. People who do not trust their abilities have no reason to live in this world. THIS IS THE CRUEL REALITY ABOUT THE WORLD.