Your earlier examples of adults becoming completely fluent were a biased subset of the general populace; the ones who couldn't learn didn't stay the necessary 15 to 20 years.
Fact: Adults can learn to speak a second language well. I gave you examples and I specified three things they have in common: immersion, mnemonics, and 20-odd years of effort.
Your argument asserts the consequent. They left because they couldn't learn? No. They didn't learn because they left.
My point is simple and very uncontroversial: There is nothing in the brain making it
impossible for an adult to learn to speak a second language well, as has been implied.
I have met very few people who've learned a second language as an adult who speak without a trace of an accent and exhibit other minor, grammatical problems.
Absolutely. But the critical period hypothesis applies to native speaker fluency. I was careful to say near-fluency. Of course, the accent will always be a little "south of the border" and they will never be mistaken for a native speaker, but they can get by
almost as well as one.
