What did Nick mean in chapter 8 of The Great Gatsby when he said, "Gatsby paid the price for living too long with a single dream," and "the holocaust was complete"?
For years Gatsby lived with the dream of winning Daisy back - everything he did, he did for her. The piles of money, the fashionable wardrobe, the extravagant house, the endless parties - all were part of his plan to finally achieve his dream. He thought the two of them could relive the past - that when she finally saw him, it would be like nothing had ever changed between them, except this time he would have the money to make her happy.
However, Gatsby "(pays) a high price for living too long with a single dream" - first he loses his grand illusion, then he watches his dream die, and ultimately he loses his life.
Gatsby's grand illusion is seen in the way he has idealized Daisy through the years and convinced himself that she has ever only loved him, that she still loves him, and that if he can only make the money to provide her the kind of life she desires, she will leave her husband immediately to spend the rest of her life with him. He conveniently dismisses any thoughts of Tom or the child she has with Tom. All of Gatsby's hopes are wrapped up in his illusory vision of Daisy, and when she can't go along with his expectation - she can't tell Tom she never loved him and that she plans to leave him and spend the rest her life with Gatsby - everything starts to crumble.
The dream dies even further when, after getting Daisy back to the house after the accident involving Myrtle, Daisy doesn't come to the window until 4 a.m., even though Gatsby had told her he would be waiting outside in case she needed him. When she does appear at the window, she doesn't acknowledge him in any way. It is as if she has already moved on. The next day, as Gatsby goes to the pool, he tells his chauffeur to let him know if there is a phone call - but it seems he doubts a call will ever come. As Nick states:
I have an idea that Gatsby himself didn’t believe it would come, and perhaps he no longer cared. If that was true he must have felt that he had lost the old warm world, paid a high price for living too long with a single dream. He must have looked up at an unfamiliar sky through frightening leaves and shivered as he found what a grotesque thing a rose is and how raw the sunlight was upon the scarcely created grass. A new world, material without being real, where poor ghosts, breathing dreams like air, drifted fortuitously about . . . like that ashen, fantastic figure gliding toward him through the amorphous trees.
Gatsby illusion and dream have collapsed. But his total reversal of fortune is not yet complete. He ultimately pays the price of his life after George Wilson shows up at his house, having been told by Tom that it was Gatby's car that ran over Myrtle. George, overcome with grief, shoots Gatsby, then himself.
With this final death, "the holocaust (is) complete." Myrtle, Gatsby, George - all are dead, and no call or flower from Daisy ever comes. She and Tom leave town for a quiet get-away, and Nick is left to say of them in chapter 9:
They were careless people, Tom and Daisy — they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made. . . .