It's the opposite, using a decentralized Youtube would save a bunch of bandwidth.
Imagine (granted, a ideal world and maybe not actually possible) a world where every routes and computer is a node that both serves and receives data. Suddenly, your ISP can start to aggressively serve traffic from their edge-nodes. If a video goes viral, your local network can fetch it directly from your neighbors network instead of reaching out to the internet to fetch it. We'll be reducing the traffic massively.
But yes, it's a ideal world and probably not possible to execute in reality as the market forces behind paying for bandwidth is so strong.
- Residential Internet connections are asymmetrical. That's a problem and renders many clients as leeches.
- It is unreliable for long tail files. If a file is never read, it will never be replicated and will eventually disappear.
- Once the universities started cracking down on BitTorrent usage, it was game over. Dorms were practically the CDNs of the torrent world.