Not all new parts can be replaced, and even those that can, Ada begins to reject. Eventually, the beautiful, smart robot begins to shake when she walks. Her memory fizzles occasionally. Her power depletes more rapidly than it used to, and she slows down.
Some things, however, never change. Alex still smiles whenever he sees her. They still take long, slow walks around the street in this world that is strange for both. Alex's hands shake when he eats now, and he leans heavier on his cane with the hand not holding Ada's.
Neither ever choose to let go first.