Another myth is that nobody is a perfect bilingual. Both of those myths hold a kernel of truth. We ignore both the degree of energy and effort that goes into generating bilingualism even for 3 year olds, as well as the fragility of bilingualism that young children achieve. It is very impressive to go into a home where two languages are spoken and watch the children address their mother and father in the correct languages. This creates the impression that bilingualism is easy for young children.
Kids can lose languages extremely easily.
Similarly, at the other end, adults who've had lots of experiences in two language environments can end up functioning extremely well in both those language environments and they might well end up being not totally authentic or native-like or convincing in any of them. There are also interactions between language and literacy. If you haven't been reading in a language then your vocabulary is shrinking. It's by reading that the lexical stores get replenished and maintained. Finally, the reality is that there are not any monolingual societies anymore.