I agree with you. I love the show. But I was pretty much out the door with seasons 6 -7. Purely because it felt like the story was told. However, with season 8, I became reinvested in the show. I think it received new life when Carver became showrunner (who, like Kripke, had a clear direction in mind).
Given the amount of meddling from both sides, the best ending for the show would have been the closing off of the gates to heaven and hell by the Winchesters (all while giving them the finger). Thus mankind would have been left to determine their own fate.