I don't think folks can learn a system effectively without a teacher. Those with a foundation can learn techniques without one. Beginners, with a good partner, can learn some basics - much more slowly and less reliably - without one. But a system will never transfer well that way. And a form picked up without feedback from the instructor will become whatever the beginner imagines it is, rather than what it was meant to be.You still think about "teacher-students" model. I'm thinking about the "google engine" model that everybody can learn MA online without teacher.
Here's a good analogy: in a lot of business environments, task-training is done without formal work procedures. So the new person takes a ton of notes during training. Essentially, they have the least experienced person in the room documenting the process. This always introduces some interesting misunderstandings in their notes (I know, because I collect a bunch of these notes when I go into a company to help develop desk-level documentation).
I've seen first-hand how badly things can be misunderstood, while following what looks like (to the beginner) the proper movements. And I don't even have to get into things as hard to understand as aiki. I've seen leg sweeps (Judo's osoto gari) done with very bad mechanics by people who were quite certain they were doing exactly what they were shown. Even working against a sofly-resisting partner, they didn't figure out those basic mechanics over time.