A server or two? How long do you expect random youtube user #3283949382 to seed a video and tie up their outbound bandwidth to help out a multibillion dollar corporation? Most residential uplinks suck, and are a fraction of the downlink's speed, even on gigabit packages. It's annoyingly easy to saturate your uplink and because TCP depends on two way communication, not being able to send ACKs back to a server cripples your download speeds. You honestly expect your average youtube or netflix user to understand p2p, seeding, and have enough network chops to add some QOS to avoid tanking their internet connection? What about hard drive space? What about the fact most internet packages have monthly bandwidth caps? So no, they would need to have enough servers to deliver their videos on a "cold cache" because they could never rely on any user seeding their content for very long, if at all. Can 1 or two servers handle all of youtube's egress if no other peers were online? No.

And we haven't even touched on the philosophical angle where undoubtably there will be users who say something like "I don't want videos about x passing through my systems" or "director so and so is a person I decided to hate I refuse to seed their content" because that would happen sure as the rising tide.

You know that a mass amount of child porn is uploaded to youtube daily right? They have teams of humans who's jobs are to review flagged videos for kiddy porn and remove it from their systems. Do you want child porn seeded from your computers and internet connection? I don't. That's a good way to get your door kicked in by the fbi. How do you delete a file shared with bt or any other p2p protocol? You can't. It's literally impossible to delete a file once seeded out there and I sure as shit am not giving any company the ability to spread child porn using my systems nor am I giving them the ability to delete files from my systems. Do you not see the security nightmare? The legal nightmare?

p2p is great but it's entirely impractical to say it can replace cdns or pops and be as reliable for the businesses serving content, because it's not. And since youtube or netflix can't depend on any consistent performance or be able to do any future forecasting since it is all entirely out of their control, they would never do such a thing. And even if they did, as an end user, I don't want to be a free seed box for any business out there, even if they are giving me ad free viewing as "payment" for hijacking my expensive internet connection, hard drive space, cpu time, and electricity.

The choices are

an unreliable and extremely complicated mess of NICs spread across the globe with unpredictable uptime, unknown security, and slow and/or flaky uplinks, that needs to honor millions of users unique personal believes, privacy concerns, fair system use requests, and opt-outs, which expose them to be distributors of illegal materials while also putting their own user base at risk of criminal prosecution...

or

tightly controlled, fully owned pipelines of secure systems in datacenters with fast nics connected literally to the backbone of the internet

There is no choice here for any business trying to keep customers happy and make money, sorry.