If it's possible why hasn't anyone implemented such a thing? I'd like to implement such a thing and I wouldn't know where to start, but I would with HLS. This is the 1990's all over again. Different browsers supporting different standards, different video encodings, different JavaScript features, sometimes, when I'm having a spell, I'm not so sure competition is actually all that great anymore.

And no you cannot stream with webrtc without building a complicated back end webrtc MCU implementation which requires significant engineering and is not at all supported by the creators of webrtc who are focused on peer to peer inside browsers.

RTSP doesn't go through firewalls easily. That's pretty much why Apple invented HLS. It's a nice, minimalist solution to adaptive bitrates that uses standard HTTP to get the job done. Big players such as the BBC have adopted the standard (ugh, I'm writing a client for it right at the moment!), so it's not a particularly fair categorisation to label this as a purely Apple thing.