It's possible they will be pulled, either licencing expired or the studio pulls it altogether from sale, like when Disney moves their movies to "The Vault." Unfortunately, there is no way to know if or when a movie or show will be pulled, nor any warning will be given.
I back up everything, not only in case something gets pulled but also to help ensure predictable performance - if I am streaming movies locally, I do not have to worry about the quality of my Internet connection or what else might be using bandwidth at that moment.
