Allow me to step in despite my vow of silence. I am not sure that immersion means in-country. Immersion refers to a school program in which the subject is taught in the TL i.e. not the student’s own. I’ve described before the Bulgarian exchange student I found standing in the middle of a group of Mexican girls, all chatting away in Spanish. When I called her over and asked her how she had acquired very fluent Spanish as well as English, living in Bulgaria (she was about 16), she said she had attended a Sp langugae immersion h.s. in Bulgaria. And she was really keeping up with the Mexican girls (I used the term Mexican advisedly b/c we had many students from Mexico at our school).
That, to me, is immersion.
Going to the country, OTOH, is a totally different proposition and the outcome would depend entirely on who the learner is. I met a Hungarian man who never grasped English; he worked in a nursery and needed little English; he was middle-aged; he was uneducated; he was slightly retarded; he never understood that English letters did not have the same value as those letters in Hungarian. So his experience of living in an English-speaking country was entirely different from, e.g. a Mexican with a h.s. education, young, ambitious, eager to speak English well enough to take classes and work in a business or start his own, etc.
Anyone with a strong desire to fit in, be linguistically sophisticated, at least enough to get jokes, curious, able to read copiously, willing to try, to err, to engage, etc., should wind up speaking the new language with some facility after 5 years or so. Not everyone fits that picture, so living in the culture the language bears is not for everyone.