My point is you could for example choose not to use very much (or any) extra upload bandwidth without getting user consent first.

That's like saying you can choose not to run a website. Like yes, you certainly could choose to do that, but not if you're planning to run a p2p based streaming website.

It doesn't have to be p2p based. That's unlikely to work well in practice. But it could be p2p augmented opportunistically.