It seems people are mixing active and passive skills as well here: For example, if you don't speak or write a language (L1 or L2) for a while, those activities

It seems people are mixing active and passive skills as well here: For example, if you don't speak or write a language (L1 or L2) for a while, those activities will become more difficult but it doesn't mean you would lose listening and reading as well. OTOH if you are separated from your L1 community for decades, you'll have grown apart and there's potentially a huge gap culturally and in vocabularies. Also, everyone has a smaller active and larger passive vocabulary, so alone it's not a sign of attrition.