This is the classical distributed consensus fallacy. Law is only law if there's consensus around it. The "original" law still lives on in the form of ETC, but it happens that the other one generates way more consensus around itself, and thus that's the more valuable one. Distributed consensus means no one can change the rules unless there's a "majority" defined by PoW, PoS, PoA, or something else, that chooses to change the rules. And there's nothing preventing anyone from proposing any kind of change and trying to get a majority behind it.

Which means there's as many "legitimate" chains as there are ways to define a majority, and as blockchain entries increasingly denote external assets things are about to get very messy. In the end disagreements will probably be adjudicated by traditional governments based on regular contract law (in the best cases) or individual physical force (in the not-exactly-so-formal ones), and after a few instances of that maybe we can be done with the whole foolishness.

This is mostly true, but I'd argue legitimacy is harder to achieve than it seems, because of network effects.

That said, the current NFT craze is idiotic. As you point out, there's lots of people trying to try external value to a blockchain using what is more or less nothing more than a "super pinky promise". And that's supremely stupid. Today's valuable blockchains have value by themselves. Linking blockchains to external properties, assets, certificates or capabilities is still an ongoing problem. Only after the identity problem is solved and standardized will NFTs linked to external things be worth anything.

So here’s a legitimate question because I haven’t seen it asked before. Whats the value of a paper “certificate of authenticity” at the moment. My understanding is that with art if you can tie it to the original issuer, then it has value. And indeed I know there is some fraud law around forging such things. So NFTs seem fairly well suited to provide hard to forge art provenance with minimal effort.

They also seem well suited for being able to transfer ownership of physical goods and also transfer things like warranties. With an NFT tied to a serial number, you could ensure only one owner with that serial numbers item can claim warranty coverage, ensure resellers don’t try and pass off something used as new, ensure stolen goods can be tracked as such, etc.

With both paper certificates and NFTs, their worth depends on the issuer. The NFT craze is stupid, but as you point out, coupling real art with a NFT certificate is a really neat use case for NFTs. A NFT minted by an author and/or a reputable source such as an auction house can act as a unique, impossible-to-duplicate certificate that helps claim ownership of some real-world possession.

It's a given that at some point this will be tested in a court, with favorable results.

Maybe the litmus test is does the object linked to the NFT have real world value without the NFT. If you wouldn’t pay for the item without an NFT, then perhaps it’s not legit.