[xyzelement]

> You seem very certain, but that doesn't seem to be the consensus about how it works.

Just so I am clear, you're saying it's not an accepted fact that people don't lose a language they actively speak?

[tashi]

Yes, to be 100% crystal clear, I'm saying the thing that feels so obvious to you that to deny it would be pure stupidity does not appear to be true.

There's a quick summary at https://languageattrition.org/use-or-lose/, from where you can get links to decades of research on language attrition. Apparently, for people who speak multiple languages, there's some kind of interference effect between languages that's very counterintuitive, which means there's not the correlation between time spent per language and fluency that you would think there would be.