I have also read that retention of a "foreign" accent by an expat may be partly psychological. It's a way of saying that "Even if I am an expat, in my heart I'm still a Home Country person."
And it can be more complicated than that. I doubt that Henry Kissinger retains a lot of affection for Germany. And I suspect that if had gone into selling used cars instead of his chosen career path, his accent would be barely noticeable. But in academia a foreign accent may be no bad thing.
