I think it's hard to bring this study to bear on the critical period hypothesis in toto since its scope is narrow in both time and grammatical breadth (just sound production, and no other parts of comprehension or production), so we should maybe leave that aside.
You're right that the critical period hypothesis is still not settled, but let's go back to your proposal, that anyone who's immersed and motivated enough will be able to acquire nativelike proficiency. While we're in the realm of anecdotes (since you make reference to your own experience), I witnessed several peers of mine in a months-long language immersion program fall behind others in the program by quite a margin, with (IMO) little hope of nativelike proficiency even with months more of intensive classes, despite apparent motivation (sometimes much more than the better performers) and 24/7 immersion in the target language.
Going beyond anecdotes now, let's consider results from the literature against the assumption that if only adults could not be so easily discouraged they would be able to attain more nativelike proficiency. In that case, we'd expect at least a few places in time and space to have second language learners who do a phenomenal job of acquiring the target language, yet as far as I know that's never been found in the literature. Birdsong[0] reports that the range of attested rates of nativelike proficiency in L2 learners is between 5% and 25%. Moreover, there's a wealth of literature that documents in detail the specific ways in which language acquisition mechanisms falter beginning around age 6[1], which is in line with the suggestion that nativelike proficiency becomes progressively harder with age.
Neither of these results pushes your suggestion beyond plausibility, of course, but it does seem to me like the preponderance of evidence is on one side. If you know of any studies with findings that support the idea that motivation can be a decisive factor in acquiring nativelike proficiency in a second language I'd be really interested to see them.
[0]: https://web.stanford.edu/~hakuta/www/research/publications/(...
[1]: https://www.jstor.org/stable/pdf/44488300.pdf?casa_token=YS2...
You're right that the critical period hypothesis is still not settled, but let's go back to your proposal, that anyone who's immersed and motivated enough will be able to acquire nativelike proficiency. While we're in the realm of anecdotes (since you make reference to your own experience), I witnessed several peers of mine in a months-long language immersion program fall behind others in the program by quite a margin, with (IMO) little hope of nativelike proficiency even with months more of intensive classes, despite apparent motivation (sometimes much more than the better performers) and 24/7 immersion in the target language.
Going beyond anecdotes now, let's consider results from the literature against the assumption that if only adults could not be so easily discouraged they would be able to attain more nativelike proficiency. In that case, we'd expect at least a few places in time and space to have second language learners who do a phenomenal job of acquiring the target language, yet as far as I know that's never been found in the literature. Birdsong[0] reports that the range of attested rates of nativelike proficiency in L2 learners is between 5% and 25%. Moreover, there's a wealth of literature that documents in detail the specific ways in which language acquisition mechanisms falter beginning around age 6[1], which is in line with the suggestion that nativelike proficiency becomes progressively harder with age.
Neither of these results pushes your suggestion beyond plausibility, of course, but it does seem to me like the preponderance of evidence is on one side. If you know of any studies with findings that support the idea that motivation can be a decisive factor in acquiring nativelike proficiency in a second language I'd be really interested to see them.
[0]: https://web.stanford.edu/~hakuta/www/research/publications/(...
[1]: https://www.jstor.org/stable/pdf/44488300.pdf?casa_token=YS2...