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.