I am thankful that my early instructors were ex-air force types who demanded and got a good PPL standard. Sometimes they seemed a bit harsh (probably thought they were still in the military) which in this touchy-feely age may not go down too well. But they always got first time passes for the PPL test. Done in those days by CAA not the school, so it was more difficult to fudge. The school probably would not have dared to put up a sub-standard student for test.
When I went on to CPL my instructor was a junior who had only just qualified and only had about 100 hours more than me. I did not learn anything useful from this guy, so after a few hours with him demanded to only fly dual with the ex-airforce pilots or I would take my business to the opposition.
When a student is not progressing, an instructor change may fix the problem and so should be automatic at a certain stage. But the really useless ones still need to be removed from the system before they hurt themselves or someone else. With many schools also conducting their own tests, this does not always happen.
One major school in the USA used to advertise 'Pass Guaranteed". A worry.