Provide a comprehensive theoretical breakdown of Nick Bostrom's Simulation Argument. Detail the three disjuncts: (1) The fraction of human-level civilizations that reach a post-human stage is close to zero, (2) The fraction of post-human civilizations interested in simulating their ancestors is close to zero, or (3) The fraction of all observers living in a simulation is close to one. Argue the theoretical implications of each possibility for our definition of reality and existence.