it's a shame multicast isn't universally supported on the Internet. It'd make it possible to broadcast a live video to millions or billions of people straight from a cell phone.
The idea of using multicast in this way also ignores some of the actual good features of HLS, in particular the ability to adapt content to a specific user's device and Internet connection.
It worked on 80+% of UK internet connections in circa 2008.
What's needed to enable multicast? Work by ISPs?
In a world where denial of service attacks are cheap and ubiquitous, my first thought is that enabling multicast would be difficult to do without reducing the cost of debilitating DOS attacks.
Say I'm an ISP with X million subscribers.
Given a peering with Y GBit of throughput to someone like say, Twitch or Facebook; with multicast, I now only end up with a single load of data for each livestream coming over the pipe.
However, this does require direct peerings. Multicast over the Internet is just not a thing. This could technically be built out with IPv6, but nobody's realistically going to arrange for globally-unique multicast space allocations and convince every AS in the world to start peering multicast over BGP for general public use.