So if the value of NFTs only exists because of a "social contract," what good are they? I could just pay the artist $1M for a normal JPEG they will send me via email (and for my money, an actual JPEG seems like it would be way more useful than a cryptographic hash of a URL to the JPEG).

In neither case do you "own" anything in a real sense, you have an artist communicating that you have exclusive rights to a bucket of bits.

The only value I can comprehend in all this is that of the ledger's utility in facilitating trading on the secondary market (since the artist doesn't need to go and vouch for everybody who eventually buys/sells the ownership stake in the bag of bits in the future).

It all feels gross to me. I can certainly appreciate that artists have to make a living, but the idea that the best way to do this is to crank out digital certificates to confirm somebody "owns" their work (but only in the sense that the "owner" can trade and speculate on that ownership stake) strikes me as bizarre and dystopian.

Since crypto trading operates on the principle of offloading to a bigger fool, this does not seem like a sufficient disincentive. The original seller and the next N people to pass that hot potato around made money.