The House of Usher fell, and the point is???

Over a cup of tea, I read The Fall of the House of Usher. Then, I wondered, where is the allegory? This short story isn't like Young Goodman Brown where the themes are seemingly endless and the moral of the story jumps out at you like an Aesop fable. To trace my thought pattern: The Usher family destroyed themselves...because...they had incest...which caused them to all die. Huh. But, the allegory can't just be "Don't inbreed" because that wouldn't be very applicable and way too obvious, no English students are that lucky. How did the Usher family really come to their demise? (I was still drinking tea, but it was now cold from sitting on the window sill for too long) They tried to defy nature. We can make tea hot, but if it is by the less than airtight windows of Virginia Hall on a cold night, it cools quickly. The Ushers wanted to keep wealth and power within their family. I propose that the house of Usher fell because the family, even with their money and power, was not strong enough to rule the natural laws of the world.