> you are "buying" nothing more than a URL in a JSON file (i.e. no legal ownership of anything),
This is wrong. I can see why you think it, but it is wrong.
The URL in the JSON file is free, as is the artwork that it points to. It's very hard to restrict the copying of digital artworks anyway, so why bother trying to?
The person buying the NFT is not "buying a URL". This phrase doesn't even mean anything! What they are buying is control of a record on the blockchain, which (with some slight hand-waving) contains a list of the signatures of the previous owners, including the original creator or their agent. While it is trivial to copy a JPEG, it is not at all trivial to copy the list of signatures from one part of a blockchain to another, unless you happen to have the private keys of all of the previous owners.
As such, the JPEG is abundant but the record is scarce. This mirrors the physical world, in which Picasso prints are abundant but signed originals are rare and hard to copy. What you're paying for is the uniqueness of the signature, not the ability to look at the content of the artwork, which you can probably already do for free on the internet.
"No legal ownership of anything" is not really true. You do legally own the blockchain record! You can decide whether to sell it or keep it, and you can generally dispose of it as you would with any other property. You don't legally own the copyright of the artwork, but as we've already established, that copyright probably isn't worth very much.
Your other statements may be perfectly accurate - I can't imagine 20M artists on OpenSea all turning a profit, and I imagine that a lot of the investors are likely to lose some money too. I just don't think that this has anything to do with faulty assumptions about how NFTs work.