single stream in the trans-Pacific backbone that gets split once it touches land, is that all correct?

Yep, that is exactly it

In that case, torrent/peertube-like technology gets you almost all of the way there!

I am also excited at peer-to-peer technology, however P2P unicast remains a store-and-forward technology, under the best of condition we're look at at least a 10 millisecond latency per hop and of course a doubling of the total network bandwidth used per node as they each send and receive at least once. Still very exciting stuff that I wish were further along than it is, but this isn't the "multicast dream" as such, which does not use "Zuck's computer" by wish I mean it does not use the cloud which is "someone else's computer". We can imagine a glorious benevolent P2P swarm that understands that it's own participation is both a personal and a public good, that warm and fuzzy feeling of a torrent with a 10 to 1 seeding ratio. But we're still using "someone else's computer" ... at "we're" using "our computer" and that's the royal "we". Multicast is all switch no server, all juice, no seed.

that can communicate with each other equally well across the global internet without any SERVERS, CDNS, or MIDDLEMEN in between, using IPv6 as God intended

Yes, well each node is a server and a middleman, but it's "our" guys I guess, of course in the real world we've now got NAT, firewalls, STUN/TURN/ICE, blocked ports, port forwarding you know all that jazz that used to put a serious strain on my router and might ending up killing "our" phones battery, plus here P2P if you're on cell your bandwidth is ratioed, some scummy ISP do not treat traffic the same way up or down, we're starting to accumulate quite a lot of asterisks here.

we are already within a factor of two of the ideal case of working multicast!

Ah no, in this case the total bandwidth use has massively increase, those users aren't communicating with multicast efficiency, they are point-to-point and those points run through the same backbones hundreds of times, coming AND going, while the sender does not have to carry that load, the internet is not a MUCH more congested place because of the lack of multicast

only so because the peertube protocol is not yet geographic-aware

I don't know enough about peertube to answer that, I suspect it's a best effort, but here I'm sure the focus is on "unstoppable delivery" BEFORE "efficient delivery"

it gets you within better than a factor of 2 of the ideal multicast dream

I'm not sure this math is mathing, we're having double the total network bandwidth per host, and it's not geography aware, it's network topology aware, a topology that is often obscured by the ISPs for a variety of benign and malevolent reasons. The worst is that the peers will cross the backbone many time, I think we're looking at a "network effect scale" in wasted bandwidth compared with multicast. "n2" ? I'm not sure, probably n2 is the worst case ontario.

without requiring every single internet backbone provider and ISP to cooperate with you

Yes this is essential, for multicast to "work" it would have to be like that. Unicast and IPv4, the internet would be useless if you had to negociate each packets between you and your peers.

considering those pipes are owned by the same types of big for-profit companies as the walled-garden social networks and CDNs that are so distasteful

Yes, I believe they do stand in the way, I believe most of the long range communication is dark fiber, which they have bought on the cheap and have made their business model to exploit and therefore NEED to keep the utility of the public internet as low as possible, that includes never allowing "actually existing multicast" to flourish.

I can easily go to any website in Japan, play video games with the Chinese, or upload Linux images to the Europeans

You can because you're in a drop in the consumer bucket, you exist in the cracks of the system. If everyone suddenly used the internet to this full potential, then we would get the screws turned on us. The internet is largely built like a cablo-distribution network and we're supposed to just be passive consumers, we purchase product, we receive, we are not meant to send.

the intercontinental pipe owners might start complaining,

yes I think so too, and they wouldn't wait for their complaints to be heard, we have been here before, throttling, QoS deprioritizing (to drops), dropped packet, broken connections, port blocking, transient IP bans, we are sitting ducks on the big pipes if we start really using them proper. Multicast would essentially fly under the radar.