Multicast group where the listener subscribes dynamically to sender. When a packet in the multicast IP range arrives in the router, they get routed depending on the subscription list. So instead of a packet being sent down one link, it might get sent down both of them, the routing rules are the same as unicast except in the destination of all hosts in the subscription list, which is only read for packets of the specific multicast destination address of the packet so not all rules (there would be a lot of them, megabytes of rules, too much for 80s computers, but trivial to route under 2 microsecond with our hardware)

I don't know the exact mechanism or if that's how it works, but it's how it SHOULD work and I think that's how IGMP actually works

I don't think the hurdle is technical, it is political, stopping all these monopolistic ISPs in between the people from trying to extract a toll for every passing packets, which of course just kills the entire concept. If it's not dynamic, fully automatic and "as free as" unicast packets then it quickly become not worth doing at small scale.

I think only government action can ram this in.