Interesting. I learned bass guitar (fretless - I don't much like fretted bass) without a teacher, and did fairly well at it. I was able to get gigs etc. and play adequately at an amateur level. For learning double bass, later, a teacher was useful and perhaps even necessary. It is a physically harder instrument and direct feedback from someone in the same room watching me was very helpful. From a teaching perspective, I have taught fencing for over 30 years. One thing I notice with many students is that they lack the ability to use proprioception. They think they are doing the thing they are supposed to do, but cannot feel that their body isn't doing it. Unlike a YouTube video I can see this and let them know, and also suggest means by which they might get to their goal. Having said that, I had one student who did quite well studying YouTube videos during the pandemic when classes were suspended. That student still benefitted from coaching later, but it was more along the lines of refinement of existing skill rather than imparting the basics. In relation to videos, these are useful for improving fencing also. Students will often be surprised to see what they look like during a bout.