People make this mistake because they think there is a "function" part to NFTs. It's a ledger, and you want to remember what media an entry refers to. You can do that by just keeping it in your community's collective memory, or you can use a hash of the file, or both.
People can then choose to value that ledger entry, or not.
Everything else, such as the ability of n app to figure out an image URL for the NFT, is just sugar on top.
NFT people will value a Bored Ape token whether the URL is valid or not - and they are entirely right to do so. The Cryptopunks don't even have a URL!
NFTs require at least one of two centralized services to remain functional - the original URL and the Opensea cached URL. So proof of ownership is decentralized, but the actual image is not.