After further analyzing the plot leak for the final season of Game of Thrones, I think I have found a deeper meaning to the bitter part of the ending. Based on the plot leak, House Targaryen will survive through Jon Snow, Daenerys, and their children. However, because Jon was never a Stark to begin with, his children cannot claim the Stark name and will instead inherit the Targaryen name. After fulfilling his purpose as the three-eyed raven, Bran Stark will die and the legitimate male bloodline of House Stark will die with him. Neither Arya nor Sansa show any signs of producing an heir anytime soon. Because Arya is so independent, she is not the kind of person to settle down and start a family. In addition, Arya was stabbed where her womb would be by the Waif while she was in Braavos, which could have destroyed her ability to have children. Also, we have no idea what Ramsay Snow did to Sansa during their marriage, which could mean that Sansa cannot have children. Overall, the bittersweet ending of the story would be that the Starks get their revenge against their enemies and save the world from the White Walkers, but their name and bloodline will die with them. This will be how one of the oldest living dynasties in Westeros would die.