When O'Brien says that Winston is the last remaining human he is saying that because Winston defies the party it shows that he making a choice between what he thinks is right and wrong in life. Winston is the last part of humanity remaining because just like the proles he loves Julia and it is not just so they can have kids like the party wants but real that he and Julia share with each other. Winston is an archetype to the revolution that he pictures because he is doing everything against the party. He needs the proles because there are so many of them that they out number the party. But they are happy with there freedom that they have so they can't realize there being oppressed by the gov.