Serious question: What happens in 20 years if the domain registration for that URL is not renewed, or the website hosting that JSON is gone?

What happens if you drop your print in a puddle and it's destroyed, but you still have the proof of authenticity in your pocket? While the artist is alive, they'll probably replace it at cost. But after the artist is gone the certificate does nothing. (It still proves your status, like the NFT, but otherwise is worthless.)

If you Google you will find that a lot of NFT art has already gone missing

Use IPFS.

This doesn’t solve the problem at all. Many of the NFT sites are hosting their own IPFS and many of the images have already gone missing from the IPFS because no one is hosting that file still.

https://twitter.com/checkmynft/status/1371633090318249984?s=...

https://twitter.com/scanlime/status/1371509988179464196?s=21

couldn't anyone else start hosting that content on IPFS, and the URL would resole to it? Since it's content-based addressing? (I don't have a great understanding of IPFS, I could be totally wrong)