If you’re OK with “ownership” being “I paid somebody to write down that I am the only person who owns this asset, and they won’t give it to anybody else”, you don’t need any blockchains. You can own the monkey picture in the same way that I own my domain name, or my RuneScape account. If OpenSea decides to change the deal, they can just do so, because while they’re using a blockchain, they are the sole entity that controls how it interacts with the Ethereum blockchain. So other than for the “NFT” hype, they may as well just run a MySQL DB with “item” and “owner” columns.
By contrast, the thing that Ethereum actually does, for better or worse, with on-chain contracts, is that the consensus of the chain is what counts. If you hold ETH according to the chain, it’s yours. The only way for another entity to take it away from you is to have your private key, or convince 51% of the participants to agree with them.