You may easily know more about this issue than me, because I haven't actually researched this.

That said, let's be more precise. If we're talking about mere fluency, there is, of course, no question.

But if we're talking about actually native-equivalent competence and performance, I have severe doubts that this is even regularly achieved. How many L2 speakers of English do you know who never, ever pick an unnatural choice from among the myriad of different ways in which the future can be expressed in English? This is something that is completely effortless for native speakers, but very hard for L2 speakers.

The people I know who are candidates for that level of proficiency in an L2 are at the upper end of the intelligence spectrum, and I also know a non-dumb person who has lived in a German-speaking country for decades and still uses wrong plural formations. Hell, there's people who are employed and teach at MIT and so are presumably non-dumb who say things like "how it sounds like".

The two things I mentioned are semantic/pragmatic, not syntactic. I know there is a study that shows L2 learners don't have much of a problem with the morphosyntax of Russian aspect, and that doesn't surprise me very much. I don't know and didn't find any work that tried to test native-like performance on the semantic and pragmatic level.

I'm not sure how to answer the "why" question. Why should there be a critical period for anything? ... Intuitively, I find that semantics/pragmatics, having to do with categorisation, is a better candidate for something critical-period-like than pure (morpho)syntax. I'm not even sure you need critical periods for everything, anyway. If A learns to play the piano starting at age 5 and B starts at age 35, I wouldn't be surprised if A is not only on average, but almost always, better at age 25 than B is at 55. Unfortunately, that's basically impossible to study while controlling for all confounders like general intelligence, quality of instruction, and number of hours spent on practice. (The piano example would be analogous more to the performance than the competence aspect of language, I suppose.)

There is a study about Russian dative subjects that suggests even highly advanced L2 speakers with lots of exposure don't get things quite right. Admittedly, you can still complain that they don't separate the people who have lived in a Russian-speaking country for only a couple of months from those who have lived there for a decade.

The thing about the subjunctive is, at best, wrong, but certainly not bullshit. The fact that it was told to me by a very intelligent French linguist about a friend of his whose L2-French is flawless except for occasional errors in that domain is better evidence for that being a very hard thing to acquire than your "bullshit" is against that.