[alatar]

I stepped right over the original question, or at least that which is posed in the thread title. "To whom was the lesson directed?"

I assume that it was to the Númenóreans, showing that that which is given can be as easily taken away. Sure, these men were rewarded for their faithfulness in the breaking of Angband, but this did not give them carte blanche to do whatever caught their fancy. When they too strayed down the dark road, it lead to their ruin as well, as it does for all beings.

Even Melkor and Sauron were good once, and yet...

Anyway, the Faithful that escaped the ruin would carry the lesson forward. Some might obscure the lesson with thought of merely the "Golden Age," but that is just part of the whole story.

Another thought: Was this a retelling of the Biblical story of the Tower of Babel, where united humanity attempted to build a tower all the way to Heaven (or however the story is read)? This act of pride was 'rewarded' with the dispersing of all humanity due to divinely-enacted language barriers, and so never again could humanity unite in such a prideful way - trying to reach Heaven/Aman.