What are the dream and the vision that Perrault reveals to Conway, the true prupose of Shangri-La? (chapter 8)

Perrault revealed that the dream and vision showed the world's most beautiful things destroyed by weapons, violence and war. The true purpose of Shangri-La was to outlive the vision.

He looked back then on his long life, as I have already told you, and it seemed to him that all the loveliest things were transient and perishable, and that war, lust, and brutality might someday crush them until there were no more left in the world. He remembered sights he had seen with his own eyes, and with his mind he pictured others; he saw the nations strengthening, not in wisdom, but in vulgar passions and the will to destroy; he saw their machine power multiplying until a single-weaponed man might have matched a whole army of the Grand Monarque. And he perceived that when they had filled the land and sea with ruin, they would take to the air. . . . Can you say that his vision was untrue?”

“True indeed.”

“But that was not all. He foresaw a time when men, exultant in the technique of homicide, would rage so hotly over the world that every precious thing would be in danger, every book and picture and harmony, every treasure garnered through two millenniums, the small, the delicate, the defenseless — all would be lost like the lost books of Livy, or wrecked as the English wrecked the Summer Palace in Pekin.”

“I share your opinion of that.”

“Of course. But what are the opinions of reasonable men against iron and steel? Believe me, that vision of old Perrault will come true. And that, my son, is why I am here, and why YOU are here, and why we may pray to outlive the doom that gathers around on every side.”