Jeez, I both survived those instructors from ages past, and was one myself. And yes, there probably were some who would have been better elsewhere, but the vast majority who taught me, and whom I later worked with, were bloody good at their job. Scrubbing a student is not an easy step, and even in that non touchy feely age, they always had 'form', on a look back through the hate sheets. Absolutely nobody was scrubbed for saying' umm', unless of course it meant they were never able to answer a question. And people are NEVER scrubbed because it looks like too many will graduate. The standard does not change; they just recruit less next time.
My course had the better part of 40 starters, and 20 or so finished. Across that 20 were a range of skill levels...not everyone who graduates is an ace, but all were acceptable. And I'm sure those differences may well still show up all these years later. Of the blokes who missed out, a few took the GA route. Two of that group were killed in light aircraft accidents, and two are now 747 Captains.