I'm not sure I see the point though?

For 1-to-1 transfers, Magic Wormhole is the system to beat, and I struggle to imagine how it could be improved.

https://github.com/magic-wormhole/magic-wormhole

What do you mean, could be? https://instant.io/ is just one of many webtorrent sites that do this. It's a wonderful use of bittorrent.

Agreed, but I don't think there's a lot of motivation to do so. At that point, BitTorrent is just DCC in a new fur coat. As stated earlier, BT is most appropriately used in a setting where you want to download a single file from multiple sources, distributing the workload between them and your network. This results in faster downloads, and does so in a way that is decentralized (for the most part) and scalable by design.

So while it could be done, the question is really "should it be done"? I'd love to be able to send a file over the internet easily without using some centralized service, but if there's only 2 people that are meant to have the file, does the overhead of BitTorrent make any sense?

Isn't this basically Resilio's Sync ? I've been using it for years on smartphone backups that otherwise wouldn't be able to hold a sustained transfer.

We already have SFTP for this.