Why is Benjamin such a disappointment in Chapter 10 of "Animal Farm"?
Benjamin is a disappointment because, although he alone among the animals remaining from the past professes "to remember every detail of his long life", he does nothing to enlighten the others so that they might stop the degeneration of social reality on Animal Farm.
In Chapter 10, most of the animals on the farm had been born after the Rebellion, and of the ones who had been there from the beginning, most of them "could not remember" when things had been different. There was "nothing with which they could compare their present lives", and so they are only dimly aware when, one by one, the principles on which the Rebellion was based are compromised. The farm "had grown richer without making the animals themselves any richer - except, of course, for the pigs and the dogs". Inevitably, the new rulers, the dogs and pigs, have become more and more like the corrupt leaders, the humans, whom they had once hated so much.
Benjamin is the only one who can see what is going on, but he is so mired in cynicism that he refuses to get involved to effect change. Benjamin believes "that things never had been, nor ever could be much better or much worse - hunger, hardship, and disappointment being, so he said, the unalterable law of life". Even though he alone has the knowledge and awareness to make things different, he refuses to act because he believes that to do so is futile (Chapter 10).