In other words, you're saying some people shouldn't be allowed to dive.
What I said was "In a well designed curriculum, students who have the required prerequisite skills and who put in the required time and effort in the amount of time designed for course completion should complete the course satisfactorily. If there is a high failure rate in a course with
properly screened students who are giving the expected effort..."
Pretty much all courses in all subjects screen to make sure students are qualified to begin the course. It is a waste of time to try to teach students who do not have the prerequisite skills needed to begin instruction. For open water classes, students are screened for medical conditions and age. They need to pass a swim test as well, but not much else. Some of that screening is done by the individual--people who don't really want to be scuba divers usually (but not always) avoid signing up.
More advanced classes require more advanced beginning skills on the part of the new diver, and doing that screening can be tricky. It is, however, important to do that screening, and one of those reasons is a matter of ethics. Let's say you charge $1,000 for a 3-day course, and you also charge for followup sessions or retaking the course for students who do not pass. If you accept students into the original course who do not have good enough skills to begin the course, meaning they are likely to fail, then I would argue you are ethically challenged.
I know of a case a number of years ago in which two students paid a technical instructor a lot of money for a full week course in Cozumel. They also paid his transportation, lodging, and food while he was there. At the end of the week they were told they had not passed, and they would have to repeat the entire course. Now, I do believe people should not pass a course just because they paid for it and put in the time, but if you have two highly motivated students working hard for the length of time you said the course should take to complete and they do not pass, then something is wrong with the screening, the design of the course, or the quality of the instruction, and all of those factors come back to the instructor.