I'm sorry if some of my personal reality offends people on this board.
This isn't really an apology. We can't argue with your reality, of course; but that you attribute the students' success or lack of it directly to the teacher's skills -- that is objectionable. It might have something to do with the teacher, but it could very well have nothing to do with him. Instead, it could be any number of things: the students themselves, the parents, geographical location, local musical culture, the atmosphere of the teaching studio, or some combination. To simply blame the teacher -- you shouldn't be surprised that the OP and others have reacted in order to defend a friend or teachers in general. What kind of response would you get if you extended this argument to school teachers? Speaking from experience, the caliber of student skyrocketed when I switched studios.
Your assertion also implies that a teacher can't improve at teaching (teachers are either "serious" or they're not, and they've always been that way), nor should a teacher embrace the idea that being a positive influence in someone's life is a part of the job, since engaging a student through gimmicks and other non-didactic material are signs of poor teaching. Some students are simply hard to reach, and the last thing you want to do is create a distaste in music for someone.
I suppose there are a few unqualified, or even bad, teachers out there, but I think it's a rarity. There's not a ton of money to be made, and without that incentive, the ones who aren't intrinsically motivated soon find other work. In the case of the OP's friend, I hope he's able to find a more satisfying teaching experience if he feels burnt out, and that the best of his students follow him to a new studio or school. Or, OTOH, that he changes up the teaching material and finds a way to engage the students. Learning to read music is difficult enough, but it's nearly impossible if you aren't motivated by
why you're doing it (for example, so that you can play that sweet fill from [Name of Song by Student's Favorite Band]).