I'm only a native speaker of English, but I know vast swathes of native Chinese speakers who have been in the US for decades and sound nothing like native English speakers. I know ones that do sound like native speakers too, but it's rare in my experience.

I don't think immersion is anywhere close to sufficient to guarantee native-like proficiency, there's something else.

I don't know what the something else is.

Have you lived abroad? Living in China has dispelled a lot of my previous notions about what is possible when it comes to language. Another person I knew moved to China after college, taught English, and ended up staying her for forty years. My Chinese friend told me that when she speaks to him she forgets he is a foreigner. It happens.

I think we're on the same page with regards to whether a non native speaker can reach native levels. It can happen with a lot of work. But time and immersion alone doesn't do it, I've already said I've seen cases where that didn't work and some where it did.