There are such things as bad students. In any sample of students (of any class at all, not just trading), a few will be good, a few will be bad, most will be so-so. Some people won't pay full attention or give full effort etc. and will blame the instructor, we've all seen this before.
Next, especially in regards to something that involves abstract problem solving, the percentage of bad students will increase as the difficulty level of the content increases. People will burn out, get weeded out, get exposed, etc. etc. etc.
Next, You're talking about a course that has no barriers to entry except money. Do these people have to pass a test first before they will be accepted? Do these people have to take some kind of prerequisite course? Do they have to have a specific set of skills that must be demonstrated in some way first? Do they have to have certain kinds of backgrounds? Do they have to have certain kinds of job experience?
There is virtually no embedded selection bias that would contribute to a higher overall "success rate" for the graduated students.
Given these dynamics, as a business, how do you make the most possible money from teaching courses?
Charge a flat fee to anybody who will pay.