[okamiueru]

I think you bring up very valid points.

The overall takeaway shouldn't be to have personal instructors in order to learn things effectively and well. I think that will always be the case. The constraint there is mostly with accessibility and availability

What you intend to learn, and whether a self-learned/video watching approach can be effective depends entirely on how to close the feedback loop, and make it accurate. For couples dancing, it's very hard to know when you're doing things right (how is it supposed to feel? What is the correct tension for proper leading? Etc). So, it makes it almost crucial to have some (semi)direct guidance in order to make the practice sessions effective.

However there are many other areas in which it is easier to close this loop, as well as more correctly assess the result. This will of course never be as good as having an expert's guidance, but the cost/benefit in terms of time spent / skill gained is more amenable that it is for dancing.

I'd say (I might be wrong) that this applies somewhat to art, technical skills like math and programming, and other knowledge based domains.

Fields requiring physical accuracy and dexterity is a bit harder. Like music instruments or dancing.