All good and valid points .... inclusive in all of these is the underlying factor, is the CONNECTION between Shop/Instuctor and Student, based upon TRANSACTION or RELATIONSHIP?
Processing students through the diver course for a fee (transactional) is hightly unlikely to create the emotional connection necessary for the student to "stick" with diving / shop / instructor. However, if their experience has in it, caring, curiosity, and a genuine interest in each student as an individual, then they will more likely see scuba diving as something they are connected to and will be more likely to contimnue. If an instructor nevers really figures out, "The WHY?", then why should they care? (Check out Simon Sinek’s TED Talk, Start With WHY, on YouTube for more on the WHY) ... it may just change how you see your students .... and yourself!