We can add the hash of the content but what happens if the URL goes 404 or the web server disappears? I'll be the owner of a useless pair of URL and hash.

Or those NFT contents (and the URL domain!) are guaranteed not to disappear unless many web 1.0 and 2.0 services people was paying for and went out of business?

An article about this: https://www.theverge.com/2021/3/25/22349242/nft-metadata-exp...