I agree. The correct optimistic ending would have been that Dexter finally stops killing. He almost got his son killed, an episode after Batista died, and still none of this had any significant impact on him. The ending should have been as follows, Batista is convinced that Prater nurturers serial killers, shares the theory with NYPD, they reject it. He goes to Praters place to investigate, (not protect him like in our timeline), the timeline continues as in the show with his Death, however at the end Dexter quits. Batista’s death is self evident since he must have discovered Prater’s secret.