Introduce a limit in the number of hours you are allowed to train before being chopped if not good enough.
Might be difficult to enforce but there's bound to be a way. Seems silly that in a safety orientated business if you have the money you can throw as much time as you need into it until you are good enough... good enough is not, 'good enough'.
Military academies around the world have systems whereby each course in the training pipeline has a set number of hours. There is some flex for students who show improvement but if you don't make the grade you are out on your ear.
Give anyone enough time and they will manage to scrape a pass...
Smith
LOFT - Line Orientated Flight Training