Big Brother does not exist. He never gets old, he never dies, nobody never saw or will see him. He takes care for Party and the proles. He is worshiped like a God. It's easier to Proles and to everyone to understand that there is someone who controls everything, one person; it's a bit complicated to understand how Party function, which was seen in way O'Brien described it to Winston. Proles adore him and hate the Party and for bad things they will never accuse Big Brother, they will accuse Party. (Similar thing was happening in USSR and China, were Mao and Stalin were worshiped by many; however, everyone hated and was afraid of secret services).
Party controls everything. You can become Party member only by born; your parents can only be other Party Members. Members are constantly controlled by Thought police to assure no one will ever doubt in Party. They don't have privacy, friends, families or emotions; everything what are they doing is for Party. One hour of privacy is strictly controlled and sex is only allowed for creating new Party members. Telescreens, Thought Police, Two minutes of hate are some instruments which Party use for control.
Party Members should never have doubts in Party's leadership. Those who have them are caught by Thought Police, tortured and there are two possible results: death or healing. Winston was healed; instead of hate for Big Brother he says: I love Big Brother!!! That's greatest result Party can achieve; and death is the latest option. That's why Winston survived.
P.S. Any comments and ideas are welcome; unanswered questions also.