Immersion is not really the only reason or even the real reason that textbooks move to all Japanese at the intermediate level. It has more to due with the market for textbooks. It costs a lot of money to pay teachers and writers to develop a textbook. It costs a lot more if you need a staff fluent in 2 languages. To produce a fairly good textbook like Genki it required a team of people with a high level of English that were also able to teach Japanese.

So instead of having to produce an E-J textbook, a K-J textbook, a C-J textbook, etc. they just produce a J-J textbook at that level, and that covers it. Anyone at the level they're targeting should be able to understand the explanations in the book.

Primarily I think the function of intermediate and advanced level textbooks is as controlled source material for teachers to use in their classes. With self study I think native material with reference books is a more effective way to go, but as a teacher it's really very difficult to find very specifically-leveled source material to teach from. It's easier to have students buy a textbook, and then have that be the backbone of the curriculum in the class.

As a teacher if you don't have a source text to work from then it becomes a lot harder to structure a semester or work toward clearly-defined goals. For classes that I've been in that haven't used a textbook it's basically lead to a de facto textbook made up from various handouts and things.

Textbooks are more an organizational tool and content source for teachers than a guide for building language skill. At the lowest levels the guide approach can work okay for self-study, but I really believe that once a student is through with beginner level they should jump into native material as soon as possible. It is a slog for those students at first, and it does take practice to develop the reading and writing skill properly. After beginner level, though, it really is all about practicing the application rather than learning explicitly new things.

I think people in general tend to look at language learning the same way they look at learning history. If only they had the information then it would be fine. It doesn't really work that way. Language-learning is more like learning to play a musical instrument. At the lowest levels you really are learning new things about the instrument and music as a discipline. After that beginner level, though, it more becomes training of muscle memory. Language-learning relies a lot on training mental muscle memory.