>>mef+(OP)
Brains don't have innate grammar more than languages are selected to fit baby brains. Chomsky got it backwards, languages co-evolved with human brains to fit our capacities and needs. If a language is not useful or can't be learned by children, it does not expand, it just disappears.
It's like wondering how well your shoes fit your feet, forgetting that shoes are made and chosen to fit your feet in the first place.