Old Major's teachings are used to form the original Commandments of Animalism. His view of humanity as upper-class oppressors to the animals, who he sees as equal to them and capable of forming their own lifestyle, is absolutely essential to his Utopian ideals.
"All the habits of Man are evil. And, above all, no animal must ever tyrannise over his own kind. Weak or strong, clever or simple, we are all brothers. No animal must ever kill any other animal. All animals are equal."
(Orwell, Animal Farm, msxnet.org)
Had he lived to see how the pigs subverted and changed his teachings, he would have pointed at their hypocrisy and failure to live up to his ideals. His idea that no animal should adopt human habits is rejected by the pigs entirely, as they start to live in the farmhouse and drink alcohol; when the pigs change the law from "All animals are equal" to "All animals are equal, but some are more equal than others," Old Major would probably reject their leadership outright, since it directly contradicts his central ideal. It is likely that he would be outspoken by Squealer, or put to death for heresy, but his staunch belief in the true equality of animals would not waver, especially as he would see how the animals now need to work harder than ever to support the pigs.