With Netflix or other streaming services 50,000 TVs is 50,000 individual streaming sessions. With a broadcast 50,000 people watching the same thing is 1 broadcast.
Multicast is effectively broadcast -- it's (theoretically) great if what you're distributing is a live video stream. But we're moving toward a world where the vast majority of what we're watching is on-demand. While I could be wrong about this, my strong suspicion is that the technology hasn't gotten better in the last decade; most implementations just don't scale to the level that a service like ESPN would require (i.e., millions of nodes reliably receiving HD video), and I'm not even sure most commercial Internet backbones support multicast.
However BT (one of the big ISPs) uses multicast to deliver their TV service, so I assume BT their connections are capable of receiving multicasts if properly configured.
That's a nice use case for multicast. Worked for me over a decade ago. I can only assume the technology has gotten better since.