The metadata only contains an URI to where the actual information lies (says ERC-721). So as long the URI stays, the hash stays. And then you go and modify the

The metadata only contains an URI to where the actual information lies (says ERC-721). So as long the URI stays, the hash stays. And then you go and modify the contents of the pointed file and lo, instead of a Picasso you own now a lolcat. Or the file gets simply removed because the hoster went bankrupt or lost the backups after a crash and you're left owning... yeah.