What could Benjamin’s cryptic answers for the other animals foreshadow?

Benjamin's cryptic answers foreshadow that the perfect world the animals fight for does not exist. In his wisdom, he knows that overworking will simply shorten his life, thus, he works as he always has and doesn't have any expectations. In the end.... we find he's right.

Old Benjamin, the donkey, seemed quite unchanged since the Rebellion. He did his work in the same slow obstinate way as he had done it in Jones’s time, never shirking and never volunteering for extra work either. About the Rebellion and its results he would express no opinion. When asked whether he was not happier now that Jones was gone, he would say only “Donkeys live a long time. None of you has ever seen a dead donkey,” and the others had to be content with this cryptic answer.