I just don't believe the theory that every single person can learn a language totally fluently if they live in a place even for 20 or so years if they came here as an adult. I've been trying to teach my dad computers for 10 or so years with no luck. Conversely, he's been trying to teach me carpentry for the same amount of time and I still suck at it. To me the concept that language can be extremely difficult for people to learn isn't exactly a foreign one (pun intended!).
I can totally agree that some will struggle much more than others. I knew missionaries who struggled with Spanish after the two years. But I still maintain that if it's something you immerse yourself in, you'll at least be passable at it. If you worked at carpentry every day for 20 years, you'd probably at least be able to make a house stand up. It might not be the best built house, but it would stand. I honestly think that if I were to decide to move to Japan tomorrow and spent 20 years there, giving an effort, I'd at least be able to speak passable Japanese.