The finale had a nice build up, I was enjoying it more than the rest of the season until the end. Resetting time shouldn't have brought Etta back to Peter and Olivia. Peter and Olivia should have never met. If the Observers would be erased from ever existing, then the original 12 would never have been alive to go back in time, and that means that September would never have distracted Walternate from seeing that his cure for Peter worked, and Walter wouldn't have crossed over to save him.
Peter and Olivia should have been carrying out the plan knowing that if they succeeded in protecting the human race, they would would be creating a time where they never met, fell in love, and had Etta. It feels like they threw it in that everything would be fine to have a happy and nice ending that didn't fit with the story. I think if they stayed true to their events the ending would have been more powerful.
I'm probably the only person left here that watches Fringe so i'll leave it at that.