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.
It's also way more efficient to use CDNs. I doubt the internet has the bandwidth for a decentralized Youtube.