Why do you think the author depicts the total destruction of the city at the end of the book

From the text:

There was a silly damn bird called a Phoenix back before Christ, every few hundred years he built a pyre and burned himself up. He must have been first cousin to Man. But every time he burnt himself up he sprang out of the ashes, he got himself born all over again. And it looks like we're doing the same thing, over and over, but we've got one damn thing the Phoenix never had. We know the damn silly thing we just did. We know all the damn silly things we've done for a thousand years and as long as we know that and always have it around where we can see it, some day we'll stop making the goddam funeral pyres and jumping in the middle of them. We pick up a few more people that remember, every generation.

In this explanation, Bradbury expands on one of the novel's primary themes, illustrating humanity's propensity for destroying itself and the hope that always follows..... the ability to start anew, and the belief that maybe one day we'll get it right.