The problem is in the scale of size of PADI - they are so large that it is very difficult to keep a tight quality control system in place.
GUE has excellent quality control because they are small enough to make refresher renewals part of the SOP. As a result bad instructors weed out or are corrected far quicker.
But the science, development and materials PADI puts out now are tremendous.
Padi does have excellent pedagogy and excellently produced and professional looking materials.
I think the question is, does PADI *want* to institute tight quality control?
I feel (subjective) that PADI will do whichever brings in more profits.
If it feels tighter quality controls will mean lesser instructors, which equate to smaller profit margins, then I am guessing it won't.
Sorry for the slight derail, but I don't think it is difficult to put in place effective quality controls. I can think of 2 very easy ways to do so, if PADI chose to.
#1. Make instructor evaluation by the trainee compulsory and confidential (online, not readable by instructor). No evaluation? No card.
Include questions in the evaluation, such as, did your instructor conduct a swim test? How many hours of pool session did you have? Have many open water dives did you have? How many minutes and depth were the OW dives? Did you instructor cover the following (list) skills in your training? Etc.
#2. Make standards for passing and failing each class easily available to the student, as a one page printout in the first page of the course materials.