An artist renouncing the copyright (as in putting it into the public domain) does not per se devalue the NFT. It is not strictly obvious if you legally need to own the copyright to sell an NFT of something in the first place (it is likely just fraud to misrepresent your authorship of the work).

The creator of a famous photograph which they have previously put into the public domain would likely find a buyer for its NFT, while you would not. NFT buyers are under no illusion about what they are buying here - it is the official sanction of the creator of a digital edition of the artwork.

Similarly, a creator could print billions of NFTs, but Damien Hirst could also sell 499 more further sharks in formaldehyde. NFTs cannot and do not set out to solve this problem.