Thanks!

You don't need multicast! CDNs effectively implement multicast, with caching, in L7 instead of relying on routers and ISPs to implement it in L3. That's actually what I did at Twitch for 5 years.

In theory, multicast could reduce the traffic from CDN edge to ISP, but only for the largest broadcasts of the year (ex. Superbowl). A lot of CDNs are getting around this by putting CDN edges within ISPs. The smaller events don't benefit because of the low probability of two viewers sharing the same path.

There's other issues with multicast, namely congestion control and encryption. Not unsolvable but the federated nature of multicast makes things more difficult to fix.

Multicast would benefit P2P the most. I just don't see it catching on given how huge CDNs have become. Even WebRTC, which would benefit from multicast the most and uses RTP (designed with multicast in mind) has shown no interest in supporting it. But I did hear a rumor that Google was using multicast for Meets within their network so maaaybe?