Old Major teaches Beasts of England and the doctrine of Animalism, then dies before the rebellion: Marx's theory plus Lenin's preserved body (his skull is dug up and saluted). Orwell wrote the character to carry the moral that the animals' misery comes from Man alone, the seed from which every later simplification grows.
The sentimental reading: the dream was pure, the successors were corrupt, and if only the right pig had inherited, the farm would have stayed free. This is the version every successor regime prefers, because it absolves the blueprint.
The structural point: a founder who dies early becomes infinitely useful, unable to object, available for any quotation. Every revolution keeps a dead man on the wall, and the wall always agrees with the living.
Is the founder's dream betrayed, or does the dream contain the betrayal?