Hmm. The "Netflix problem" is definitely an issue I hadn't considered. However, I suspect that continuing to use P2P wouldn't actually result in reduced costs

Hmm. The "Netflix problem" is definitely an issue I hadn't considered. However, I suspect that continuing to use P2P wouldn't actually result in reduced costs in practice. P2P is complex, and getting a reliable streaming service out of it is obviously a more difficult technical issue that "steaming some bits from a server." Indeed, I'd expect that with suitable peering and Netflix-style caching servers within ISP networks, it wouldn't even save ISPs money.