Students don't need to know everything a teacher knows.
A long time ago, we knew this. I read an interview with Phil Dunlap years ago where he explained that, in the system he teaches, it was only the heirs to the system who learned the whole thing. Everyone else was supposed to learn one or two of the subsystems, based on the teacher's evaluation of their personality, body type, and so on. My understanding is that Okinawan karate was often taught in a similar fashion: the teacher would evaluate the student, teach them a kata or two (plus applications) based on what the teacher felt would work best for that student, and that would be it.
There was no need to teach the student the whole system, because the student didn't need it. Indeed, knowing it probably would burden the student with some extra baggage they didn't need in a fight, leading to Rory Miller's "brown belt syndrome".
But in the modern world, where we teach martial arts to people who want to study it for a lifetime, who train, not because they need it, but because they think it's fun, we have to give them a reason to keep coming back. So more and more of the curriculum gets shared, until people who were never supposed to be teachers are learning the teacher's system.
I don't know if there is a solution, or if a solution is even needed, but it's something I've been thinking about.