you are making the assumption that the original BTTF timeline is Our world. How can we have perceived the events from BTTF2 and 3 unless we share the same or a divergent timeline from Marty's ultimate final timeline? By your initial assumption, Doc died and Marty and Einstein vanished from existence. However, because you have knowledge of the divergent alternate timelines, the only source of knowledge of the original timeline comes from Marty's existence in our realm.