> The platform disappears, or bans you, or changes policy, or is blocked, or whatever, and your NFT is back to being a bare-bone blockchain contract number and a token id.

So what? You can obviously right click to save the JPEG, so if the platform goes down it doesn't really matter. What you're buying is ownership of the NFT of the JPEG, not the JPEG. Even if the platform goes down, you'll still be able to see the name of the piece in the metadata.