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.
That's a very impressive achievement, but I think this is one of those classic examples of the post hoc fallacy, "I succeeded by doing this, therefore those who do this will succeed." That logic is unsound.
Yes, that logic would have been unsound. It was not my reasoning, just a data point, and not a very impressive one :)
Deep enough conversations, periodically, prevent language attrition. How can I convince you? It's really just like anything else: practice helps. Is that a controversial claim?
Just so I am clear, you're saying it's not an accepted fact that people don't lose a language they actively speak?