Because of Napoleon's innate understanding of mob rule, and his careful destruction of the original Commandments of Animalism, it is unlikely that there could be a new revolution among the animals. Napoleon keeps an iron grip on the animals through thought-control -- using the sheep and Squealer as his mouthpiece -- and through fear, keeping his trained dogs ready to kill any animal that he believes is plotting against him.
Sometimes the older ones among them racked their dim memories and tried to determine whether in the early days of the Rebellion, when Jones's expulsion was still recent, things had been better or worse than now. They could not remember.
(Orwell, Animal Farm, george-orwell.org)
Since no one remembers the old days, or the original laws, they accept the rulings of the pigs without question. Only Old Benjamin, the donkey, claims to remember the old days, but because he is so resigned to the inherent misery of the world, he is not likely to promote another revolution. Napoleon's control is complete, and since he utterly destroys any hint of rebellion, his rule will likely continue.