I disagree. It is going to take longer in immersion without grammar than with grammar. Knowing grammar helps you know what to expect and helps you form your sentences because you know what sort of things to put next. It lets you pick up on these things way faster, and the faster you know these things, the sooner you make habits of it.
It isn't like you think about grammar forever.
And by the way: I had formal language training, for adults. Immersive, even: I am in the target country, spent 15 hours a week in class with other immigrants, and was taught by a native speaker. Most of the learning was introducing new word groups and practicing speaking and listening. A little bit was grammar, though, for specifically the reason I listed.
It isn't like you think about grammar forever.
And by the way: I had formal language training, for adults. Immersive, even: I am in the target country, spent 15 hours a week in class with other immigrants, and was taught by a native speaker. Most of the learning was introducing new word groups and practicing speaking and listening. A little bit was grammar, though, for specifically the reason I listed.