The sinking of Numenor was sad, because it shouldn't have happened. Numenor was once a high, mighty, light civilisation; the perfect human existence. That they fell from this incredible enlightenment and glory to rebelling agaist the Eru and the very Valar who had provided them with their island, and to being jealous of the Elves who lived forever, is the tradegy. Their path had turned astray.
Thir destruction was certainly necessary, for they were an intolerable danger to the world and were now, in fact, an evil power. This is not to say that the fall and destruction was not lamentable.
And no, I don't think it was a 'lesson' to anyone really. It was just a necessary act; although it probably put the fear of god (The real fear of god) into the Dunedain forever. I spose that was good outcome anyway.
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