It's really not. Intonation, posture, attack, phrasing, proper breathing... technique in general for singing or any instrument cannot be taught without being right there with the student. If you are at all serious about learning to play an instrument, YouTube and MOOCs are not a solution.

I think it's a rare person that can effectively teach themselves anything requiring precise motor control without a coach to watch and give feedback.

Doable? Maybe. Effective for the general population? Unlikely.

I don't believe it's even doable. Could I pick up an instrument and learn to noodle around on it from watching YouTube videos for a few weeks? Possibly, although for some instruments I doubt even that's possible. But to attain basic proficiency, much less the level of mastery required to play at a professional level just isn't possible without someone to show you how and to correct you when you're wrong (and thousands and thousands of hours of practice, of course).

I teach myself skiing, I am a qualified coach as well though.

Am self taught at computing too.