This is also part of my point, e.g. having learned Italian for years was exhausting and mostly pointless when I did it, and nowadays I can somewhat understand it whenever I am in Italy, which is maybe every two years. Was that worth it for the level of skill I gained: probably not.

But I wouldn't be able to tell if some of the mental agility I have today (beyond languages) could be attributed to the fact that I was exposed to this language which differs from my first languages in multiple ways. The lessons you may take away from any given subject is not necessarily limited to its raw content.

This is a quite known topic in educational sciences, e.g. when answering the question why it still makes sense to teach kids how to do maths without a calculator in an age where you have it always in your pocket — that is because it (A) forms an intuition about mathematical correctness and (B) logical and numerical thinking gets trained as an side effect.

Ofc all of that doesn't happen when you are unlucky enough to have had bad teachers, but that wasn't the argument.