I give lessons for a living, and my students know what it took to get to my level... I actually give lessons to 2 or 3 people at the same time, because it encourages each of them to go on their path along the others...

I actually think that a lot of the lessons come from the student, from the questions he asks, what he wants to learn... Eventually, I'll slip what I judge necessary that he knows in one of the lessons...

As for 'a lot' that you mention, 90% of the people that pick up the guitar give it up in the first year of practice, and even though your argument is true, I do not think it's the only reason why people give it up... I tell my students that the instrument gives you what you gave him... You'll master it if you've put in the time and practice, and the discipline it requires... And it is known that people have no time, that practice is boring, and that lots of people lack discipline...