You bring up a good point. It's interesting that YouTube at least doesn't do p2p for their non-DRM content.
Sounds like a recipe for dissatisfied users
"Why's my internet slow? Oh, YouTube is uploading a bunch of stuff to other people"
"How did I hit my bandwidth cap for the month already? Oh, youtube is..."
But bandwidth is extremely cheap in 2025 to the point that I do not even check my bandwidth usage and I have never reached my 2TB/month bandwidth cap in the last 3 years.
Secondly, the p2p system will be advantageous for the videos that most people watch, i.e., popular videos. This implies that the "popular" video will have a large number of concurrent users who are transmitting a small part of video to just 3 other peers who are then transmitting the same part to 3 other peers.
This way, the bandwidth usage for uploading is reduced.
Those problems are implementation specific
I don't see how you implement p2p without the p2p part.
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.