Corwyn
I really can't speak for other arts. Maybe someone more knowledgeable could speak up.
From what I've seen and read some styles designate one student to be the lineage holder and everything is passed on to him. He then becomes the head of the system when his teacher retires or dies.
Other disciples may get the complete system (or not) but only one is designated top dog. In the case of Wah Lum this person is Master Chan's daughter Mimi. He also taught in a way that only she will get it all. Smart move on his part.
Master Chan keeps tight control on the Wah Lum system. You will not find a Wah Lum teacher that didn't come from his organization.
As far as good skill and being a good teacher, they are two different things. That's why you should look at the students, not the teacher, when evaluating a school.
Whether Mimi is a good teacher or not never entered the equation. He's leaving the system to family, good bad or indifferent. His choice, he can do what he wants.
Master Chan has organized the system and added a lot to make it what it is today. In a lot of ways it's his system which is based on Lee Kwan Shan's original creation.
In any system each generation modifies the teaching in some way. In most cases it's expected. In ome cases it changes enough that a branch style splits off and goes in a slightly different direction. That's how the different styles of Northern Mantis got to where they are today. So change is not neccessarily a bad thing. It adds diversity and new ideas. Without change the style stagnates and dies out.