What does Benjamin mean by saying "Donkeys live a long time. None of you have ever seen a dead donkey."?

Benjamin is one of the animals who reflect Orwell's views. He has seen history repeat itself under many guises. He isn't excited about the revolution because he has lived so long. He understands animal nature (really human nature) and sees that it is innately flawed. He seems to know that an animal utopian society can't exist because stronger animals will always covet power at the expense of weaker animals.