I'm yet to see the movie, so it's hard to talk about Dersu's case, hehe. But I just want to say briefly that I think that we are more part of the environment than we usually think we are. Eastern folks realize this better than we do (cf. Art of War as a lazy example). It's not easy to switch environmental habits in our minds in an instant, especially if we are forced to do so. So I guess that Dersu could indeed be depressed in this sense that there were no familiar anchors to rely upon in his environment, and also many of "urban customs" often seem plain stupid to rural folks, often for good reasons!

OTOH, I remember one document about a painter in which he states that he is a 100% potted plant in the sense that he belongs to the city. He didn't miss his rural roots. I also remember my grandfather who moved to a city for work reasons and he said that living and working in a city is a step-up comparing to his past life.

As you say - this is a complex and multilayered thing. I guess Kurosawa was intelligent enough to see all those shades of grey in his movies and perhaps that's why he is considered a legend, and many rural youtubers who try to push some kind of "romantic illusion" are not :)