Final answer:
The size of the Netflix DVD-by-mail customer base was critical to repelling rivals because it allowed Netflix to negotiate better deals with movie studios and generate valuable user data for personalization.
Step-by-step explanation:
The size of the Netflix DVD-by-mail customer base was critical to repelling rivals because it enabled Netflix to offer more attractive subscription packages. With a larger customer base, Netflix had more leverage in negotiating better deals with movie studios, allowing them to secure a wider selection of titles for their customers. Additionally, Netflix's large customer base generated valuable user data for personalization, which helped them tailor their recommendations and improve customer satisfaction.