The most pressing significance of Old Major's predication is that he is right in the most bizarre of ways. In making his argument about how human beings perceive animals, Old Major suggests that Boxer will be used by the human beings until he has no more use and then he will be sent off to the "knacker's," which is another way of describing death. Old Major states this to bring to light how the animals have to rise up and assert autonomy in construction of their own world in order for them to have control in their lives. Boxer is motivated by this message and believes in it and Old Major's articulation of Animalism.
When Boxer's lung collapses due to his excessive work habits and his age, he is no longer deemed effective in working. He is sent to the knacker's. However, it is not the humans that send him off. Rather, Napoleon and the leadership of the pigs seals Boxer's fate. Old Major is right, but it is not the humans that kill of Boxer. Rather, it is his own animals that kill him off when he is no longer of use. Old Major's prediction is correct when he is describing what will happen to Boxer and in describing the overall mistreatment of the animals. Yet, he speaks from how authority will always mistreat those who are subservient to it. The leadership of the pigs proves that it is not animals or humans that is the critical point of differentiation, but rather who has power and who is unafraid to use it.