I’d say ops statement is false from direct experience of diaspora. It’s common enough to forget your mother tongue or at least relegate your competence in it to a second language.
Lots of foreigners try hard to fit in. It has consequences.
Lots of foreigners try hard to fit in. It has consequences.
Even then, attrition doesn't result in complete ("true") language loss. First, there is a decline in vocabulary, and eventually a loss of accurate phonology so that native listeners no longer judge them to be native speakers. However, they generally retain the ability to comprehend and communicate in their native language, even if they haven't used it for many years.
The point is simply that it is VERY difficult to completely lose your native language as an adult, assuming it was your dominant language throughout childhood.