The use of NFTs for tracking subscriptions, memberships, or other credentials is an interesting idea in its own right, but let's not confuse the issue by claiming that these external services keyed to NFT ownership are somehow "inside" the NFT. They're a hybrid of centralized services with decentralized access control. Everything "inside" an NFT is a matter of public record on some blockchain; the nature of the system precludes storing any hidden information. Ownership of an NFT can serve as credentials for accessing information on some other server, but then you're forever dependent on whoever runs that server.
Of course if you're dependent on a particular centralized server for the content anyway then that server could just keep track of the ownership itself and avoid the overhead of a blockchain, but there could be cases where multiple services want to share the same access control in a decentralized, market-oriented way.