[higherpurpose]

> so long as there's a large enough audience for each video

Completely a non-issue. The providers would "seed" all the videos from their own servers, too, so when there aren't enough peers seeding it, the bulk of the streaming would be offered by the provider.

Using the Pareto principle, something like 80 percent of the videos would be "long-tail" with few to no seeds (other than from the provider), but only use 20 percent of the video traffic (so the provider's burden is greatly reduced anyway). The other 20 percent videos would represent 80 percent of the traffic, and those are the videos that would be helped most by P2P streaming.