I want to add, while I suppose in reality people would settle for specific blockchains (like you can identify the Bitcoin blockchain), I can see a problem for NFTs. As long as there is not the ONE blockchain for NFT ownership, it would be difficult to prevent somebody "selling" the same thing on different NFTs.

Even if the seller says "xyz is the official blockchain", it does not really solve the problem, as they could just say that several times for different blockchains. It does not actually add new information (signing something and putting it on blockchain xyz is equivalent to signing something, putting it on blockchain xyz, and saying "xyz is the official blockchain for my NFTs").

The people decide which blockchain to use.