You are assuming all the peers start at the same time. It would be more fair to assume that the peers start regularly. In this case, the order does not cause issue.
But then you are assuming that people will seed for an considerable time (more that a few minutes) after watching the stream. If as many people do they do not seed then the last block will forever only have one seeder. (which would at least be the case for the last scene of the last episode of the last season, a point where you might not like buffering)
There is a fundamental assumption underlying this topic, are the user expected to be on desktop or mobile (including many laptops)? If the objective is to provide high bandwidth then mobile users might see a significant increase in battery and disk usage.
not saying it would not work, I actually like the idea of more peer to peer networking. But as the market is clearly focused on low consumption user devices with commonly little drive capacity seeding would be a damage to UX.
