Your command and understanding of the grammar of your native language puts a hard limit to how well you can learn other languages. This has not been stressed enough and schools have all but given up trying to teach children grammar because as natives they more or less get along without it.

On the other hand, I only learned (my native) English grammar by studying another language. I mean, I used standard English intuitively, but couldn't have told you any of the technical terms. I agree with modern educators that explicit grammar instruction beyond a very, very basic level should not be a high priority. Exposure to and guided close reading of complex texts sharpens grammatical intuition, right alongside all of the other benefits of an advanced reading level. Knowing deep grammar does not so automatically improve textual interpretation.

This is speculation, but I wonder if the period of emphasizing explicit grammatical instruction wasn't an accidental interregnum. That is to say, back in the days when Latin and/or Greek were part of the ordinary curriculum students learned grammar much as I did, as a "natural" excelerant to interpreting a foreign tongue. Once those languages were dropped educators noticed students couldn't do grammar analysis anymore, and so tried teaching it directly, without fully considering when and why it might be useful. I don't know how well the dates line up, but it would be interesting to look into.

This. When I first started learning Russian, we immediately jumped into basic grammar rules. After two weeks of incredible frustration, I realized I did not have sufficient mastery of English grammar to be able to establish a framework for understanding Russian grammar. I often say that my first two months of learning Russian were spent learning English and it is not a joke.

in all countries where i lived, schools where I studied, there was heavy investment in grammar. (no, i didn't study in usa).

I won't really agree that mastering grammar of native language limits on how well you can learn other languages. Maybe it matters in the way how it taught in college, when you are older and approach to learning language is "more structured". But when I learned Georgian at age of 6 and Hebrew at 12 (through very deep immersion. Teachers spoke only Hebrew), English at 14 (I had 5 months of private lessons following by dial-up connection to mostly english internet), it didn't matter. At least not for me.

There was also this interesting phenomena, that immigrant when they went to local school, their scores in hebrew grammar classes were usually higher than those of native speakers.