DVD has been around since 1995 and is still selling strong. Blu-ray isn't going anyone. DVD's/blu-ray/4k will continue to coexist for the same reason soft and hardcover books are both sold, some people want the cheaper one, some people will pay more for the nicer one. Physical copies is a good way to ensure you own the movie, if a studio gets bought out or is shut down, you could lose all your digital movies through them. I'd personally recommend blu-ray over 4k for a few reasons, but the decision is yours.