Those services are for the initial peer discovery and SDP exchange, which can be done by other means (using any other side-channel).

Ongoing work is happening, for example by the webtorrent folks, to remove this constraint.

Yeah, it's initial peer discovery and connection setup (if behind NAT) I'm talking about. Once you've found the other PC and got a connection set up then all is well ;-)

Do these services also act as connectors in case of double NAT?

STUN allows all sorts of hole-punching for cases where both peers are behind NATs. For cases where the hole-punching doesn't work (a user behind two NATs might be one case), you can use TURN which is a dumb UDP or TCP relay.

They certainly can, I don't know if they do in practice. It always work for me, so I haven't investigated too much.

Which is a step backwards compared to bittorrent, since that can also provide decentralized hole punching, bootstrapping and peer discovery and not just the bulk data transfer. Plus being implemented at the TCP/UDP level allows more fine-grained control over the data flow, making it more friendly towards the network.

They're also needed for the actual javascript code that runs everything.

What about something like Tor. You can run a hidden service to get an onion address that any Tor client can access.

Would that meet the requirements without being a "central" service?

If privacy isn't your primary concern, and this is absolutely not private, you can also just use a redirection service and access .onion domains without installing Tor.

Just take any .onion domain and append it with .to and it should work. examplelettersblah.onion becomes examplelettersblah.onion.to and that works without needing to muck about with Tor.

This would be a horrible idea if you're concerned with privacy/anonymity. But, it'd make it easy for people to download your shared files.

That's exactly how Onionshare works. https://onionshare.org/

Never seen that before. But running a hidden service on Tor is absurdly easy, then you just shove a file server through whatever port Tor is forwarding.