You’re right and wrong at the same time.
You’ve made one very accurate, powerful observation, which is that grammar instruction does not, by itself, result in accurate language use. No matter how much grammar you’ve studied, at street level, for actual use, you’ll start to pick up the language in phrases, or what experts call “chunks” nowadays. With time, and more exposure to natural language, these “chunks” get broken down into grammatical language. So in real learning in immersion, the process operates in just the opposite direction from the way they teach it in language classes.
However, I disagree with you in the belief that the grammar materials are junk. The grammar materials make you conscious of the structures in a language so that you will notice them when you hear or see them. This noticing cycles into your own language. For example, most foreigner learners of English would need another lifetime to figure out when to use the present simple and present continuous tenses if they just listened to people speak or read books. Explicit grammar instruction makes them aware of the difference between these structures, so that they can notice it in their environment and begin to imitate them.
Personally, I’d recommend you study grammar materials and at the same time read a lot and listen to the radio or watch TV.