But where you say “you’re stuck with what you got” is controversial and (can be ) harmful

Children can differentiate between two different languages from before birth. Children can grow up learning two or more languages, which makes it much easier for them to learn languages later in life. Learning a second language impacts the first. A childhood native language can fall out of use in adulthood and a non-native language can become dominant. You do not have to be native level speaker to be entirely proficient In a language. (Think of people from the Netherlands speaking English) This critical period you reference, as well as this “cognitive mechanism that you used to acquire language”, is heavily contested and has more to do with the effects of language on cognition than linguistic ability or competence .It seems like you are devaluing language that is learnt at a later stage in life, this upholds an out of date view on a commonly misunderstood process that is not actually true and can be harmful.