Just for the record, I did read the whole post
I actually originally wrote a longer thing about imagining yourself as the casting director and then a whole paragraph wondering if anyone in their life actually told them or if they'll find out in 5-10 years or if they're just off key in their videos, when it could've been a "I wonder if anyone actually told them", but felt like it was detracting too much from discussion.
Genuinely, I apologize if that offended you, frankly I just thought it felt a little uncomfortable that your example seemed to be of someone that you don't even know for sure if they've been made aware of their flaws. Posting a less than good video of their singing or trying auditions doesn't indicate that they're sheltered from opinions, just that they're ambitious enough to want to try anyway.
Since you also brought up the art school thing, I only called that out because your statement seemed quite "broad strokes" and seemed to miss the real reason why art degree majors can have below professional skills: wide variation of quality and teaching philosophy across art programs. In my, admittedly only 2.5 years experience, with art and design in college, a good art program will have professors that will definitely give you a big roast and won't leave you naive to your own lack of skill, and your grades will also reflect that. People have failed and had to repeat courses. The sad thing is, there are a lot of art schools that are basically just degree mills and won't even teach you the "potential" of becoming good. It is harder to detect than say, science schools because in science, there's a clear right or wrong answer to things.
All of this was what I thought about reading the post, but at that time I figured it wasn't that relevant to the discussion so I didn't expand on this at the time and wanted to keep my feeling on that light. I much more wanted to tackle the actual question in the title, which seemed to be a question on how to make someone aware of their lack of skill.