consumer routers are not the main problem with that (they might even support multicast, because various ISPs use it to distribute TV channels) - the public internet generally has no way of handling it.

WebRTC can support it all they want, but to a first approximation, Multicast doesn't work over the open internet, so it won't help outside of corporate videoconference (which generally runs OK, because corporate networks are mostly wired and often overprovisioned or easier to provision than homes)