I dont think immersion is overrated. If you know how to take advantage of your situation i think immersion is one of the best ways to progress fast in the language you're studying. But immersion alone is not enough at all. You need motivation to learn , you need to put in many hours of study and only then will immersion be worth it. Most expats will learn the the sentences they need to get by and if they spend enough time here through immersion they will naturally learn a few things here and there. But without any deliberate study or anykind they will not just Absorb the language by sitting on a park bench and listening to people without any supplemental studying.
I've been studying korean on and off ( mostly off) for the last 5+ years. When i look back at it all, I havent studied much and it probably amounts to mostly Listening practice ( thousands of hours) and then some reading , few months on lingq , some speaking ( on a very very basic level). All that shows in my knowledge of the language and ON top of that I've been living in Korea since last year april and I've barely done any studying since. I can say that the listening has come in VERY HANDY and that even now I'm picking up words that I've studied a long time ago but out of no where i hear it and it just "clicks". Me being immersed has Helped with my korean and I've got a huge passive vocabulary. HOWEVER don't ask me to have a conversation other than for basics. Words just don't come out of my mouth and it has nothing to do with pronunciation because my korean pronunciation is quite decent for a "foreigner". Its more to do with the lack actually putting in time into the language.
Anyways, Generally speaking most expats that come teach English in Korea have absolutely no interest in the language. They come here short term. The ones that stay here longer and still dont learn probably either have no interest in the language and they can get by just fine without Korean OR they just don't have time to study ( like me) not enough motivation or they keep putting it off for 9+ years and they just give up.=) In my case, other than listening to korean, I just haven't had ANY time whatsoever to actively study Korean. I know i could become fluent in the language if i put in the time. But thats a big IF and unfortunately I just dont have the time. I definitely need to sort out my priorities and start putting in the time because i'll be living in korea for a while longer....
I can also see that teaching english in korea ( which i don't do) you get paid to speak english and I can see it would be easy to only surround yourself with other english teaching friends after hours or just going home and watching your downloaded english tv shows and then 5+ years has passed and you still dont know korean past a tourist level.
Also, I think theres a good number of expats that do put in the time to learn the language even just for fun or trying to meet people. I wouldn't go by the "shocking" reactions you get from koreans of you learning their language... Koreans are "shocked" at some of the most basic things such as how we can eat kimchi, that we eat spicy food, that we know how to hold chopsticks, that we dont eat hamburgers everyday etc... I think in general most ( some dont...) expats do go through the phase of trying to learn the language even at the most basic level. The issue is ... theres probably a tiny tiny tiny tiny microscoping % of expats that actually do end up learning the language to an intermediate + level and I'm sure this carries over to anyone who tries to learn new language. Most people end up giving up and dont make it past a beginner basic knowledge level.
soooo umm yeah immersion is good for you but just because you live in the country and you're surrounded by the language 24/7 doesn't mean you'll learn the language. Expats are great at creating bubbles around themselves, putting on earphones to listen to english music , english podcasts , watch english tv etc.. Bascially block out everything. Theres more to immersion than just being in the country and surrounded by the language. You actually need to put an effort ...