Why do we have such a nasty feeling about classrooms?
I don't! I've studied about 15 languages in university classrooms, some for several years. There's also nothing wrong with doing this in addition to immersion (in fact, the guys in the video did that-- not classrooms specifically, but more or less explicit instruction via audio/software training). In fact, I am generally opposed to classroom-based immersion/"conversational" approaches. I like explicit instruction.
But... it's an unfair comparison to children, and if the goal is to become a native speaker, I imagine it would look much more like what they're doing.
My point isn't that classrooms are bad, but that I don't believe they're likely to disprove that adults can't become native speakers. If we want to test that hypothesis (either way) we'd need to have a fair comparison-- that would look more like this. You can't just take a classroom of ESL students and say "wow, they're not native speakers, let's assume adults
can't ever become native speakers".
That's all I mean, just from a theoretical perspective. (As far as I am concerned, unless it is for pedagogical purposes, quite a bit of L2 research is flawed in using classroom-instructed adults. That isn't always appropriate for some of the questions being asked.)
It depends on what you are able to do in them. I remember many years ago we made an informal experiment. We had during a month one of our students in a foreign place with no tuition whatever; and we had another, a student, with formal training over a month on that very same foreign language.
Well, the results were amazingly clear. The student in the classroom could do a lot better than the other in the foreign country.
I do not claim any value in this "experiment", but it was so striking that we were able to make comparisons intuitively and decided that classes could be a better start
That's immediate performance, not ultimate attainment. I don't think there's any question that explicit instruction is: i) more effective short term (faster); and ii) less awkward/exhausting to go through.
So in the end, I don't know that I'd recommend doing 3 months of total immersion to someone just because they want to learn a language for practical reasons, but I at the same time I would like to do research with some people who are doing that, for the purposes of linguistic theory.