There are companies doing something like that (StreamShark, Quanteec, Eyevinn; even Cloudflare with WHEP). On a company I worked on, they used Eyevinn for events with more than 100K users; and there were still performance issues.

Besides bandwidth problems (as you can't 100% rely on remote connections), any P2P solution would mean the same fragment will be shared many times between clients; something CDN networks have solved already (just serving content, instead of juggling with signalling)