how would the meaning of the book thief change if the narrator was just another unnamed human instead of death

Death as a narrator is imperative in this book. He (we think) serves as a reliable and objective source of information through the chaos that surrounds the characters in the book. It would be a jarring lack of continuity to change the narrator at any point in the story. Our metaphysical anthropomorphic guide serves us well. He also ties the reader into the story well because, after all, we all will meet him one day.