THIS. The closest I ever came to immersion learning was the semester I spent in Italy in 2004. (I was an art major studying Renaissance and Medieval art history. Absolute score of a semester.) They taught us Italian in the classroom, and we had to use it on the streets in the town we were living in. I still remember a handful of phrases and words that got a lot of use. By the time we left, after being there almost four months, I was on the cusp of being somewhat conversational. I think if I'd been there another month, it would have clicked.
And then I came home to America, and knew absolutely zero people who spoke Italian, other than my college roommate (who was on the same trip), and she wasn't fluent either. And, like, why would we talk to each other in our limited Italian when we were both native English speakers? So yeah... it's been around 17 years since then. I know fragments of it are still in my head, but to say I'm rusty would be a gross understatement.