Synecdoche, New York is not about someone on a dead end journey, it's the exact opposite.
It's about a man's journey that leads toward the implementation of his shadow. Sammy is his shadow, most obviously when he jumps and then Caden stands over him saying that he didn't jump. This is the moment Caden realizes the difference between his true self and his shadow. The movie is mainly a metaphor for a Jungian journey through life. Implementing your shadow can get you out of a dead end life. Adaptation shares the same themes. Becoming your higher self can change your life.
The film follows a trajectory of Plato's cave, out of the realm of shadows (or simply put, false ideas about the world) into enlightenment. Anyone who has been through this journey can relate the rebuilding of a city over and over again to the real world phenomenon of understanding one's life through many different angles and perspectives. If this is done correctly, the understanding of all the parts of one's life and then understanding life as a whole, which is the end, leads to self-liberation. This liberation is mostly freed from personal suffering and opens the gates for great creativity to flow.
For a movie that can truly induce existential dread about not moving forward watch Jeff, Who Lives at Home. He's basically trapped in the hamster wheel or the rebuilding of a city over and over because he's, by unfortunate chance, too stupid to escape it. He finds a false salvation in one event rather than in an implemented life path.