There are two main reasons why Netflix downloads have to be expired. One is that it's required by the license agreementS and DRM restrictions to protect movies and shows from unauthorized reproduction and piracy. The other is to ensure everlasting revenue. Thus, subscribers have to renew their subscriptions to enjoy long-term offline playback rather than download the titles once and for all.
All Netflix downloads are tied to DRM protections and are eligible under an active Netflix subscription. If you cancel the subscription, the downloads of Netflix movies and episodes will be deleted from your device when the subscription expires.