Earlier quoted context omitted.
Vimeo works with youtube-dl. Probably VLC. Video viewing (as opposed to previewing) really belongs outside the browser anyway.
I'm a heavy YouTube consumer and this just seems like a headache with extra steps/complication, while shooting features like live streaming + chat in the head. Also it's a normal use case to view videos in your browser.
Using mps-youtube to search, curate, manage, and play lists of videos, without relying on YouTube's server-side management is a real bonus.
Hooking mpv into the stream more smoothly would be useful, and I don't have a good method yet. But given my primary use-case (listening, audio-only), the fact that in the case of video and audio you really can only watch/listen to a single performance at a time, and the fact that in general what's most useful for me is to have a queue of content that I'm going to watch/listen to, in sequence, non-browser tools turn out to be far more useful than browser-based ones.
Most especially when you throw auto-play and advertising into the mix, and find yourself with multiple auto-playing tabs amongst ... well, I'm embarassed to say just how many browser tabs I typically have open, but it's a lot.
WFM. YMMV.