It may well me that buying tweets is stupid, and that no one will ever again pay even a dollar for the NFT of that first tweet, I don't know.
Nevertheless - it doesn't matter if Jack Dorsey deletes his account, or even Twitter shuts down. The tweet cannot disappear in the same way that all historical events continue to be a part of the human story. It sure would be nice to have a copy - the Internet Archive probably will, and surely others. But the fact that Twitter existed and that Tweet existed will never stop being true, and once the website disappears, nothing changes for the person who owns that NFT: There are no rights that he loses, nothing tangible that disappears for him.