That's true, well said. I had forgotten about his thoughts on suicide.
I also remember that it is made clear that In the future Watanabe purposely distances himself from Nagasawa and is disgusted by him after his letter about Hatsumi's death, so perhaps he has strengthened himself over time.
However, I still just find it interesting that the book ends the way it does. It makes me draw similarities between Watanabe in Reiko, as she seems to be a very strong and independent person, but is prone to complete mental breakdowns something rattles her world. It does appear that she has regained herself by the end of the novel, but it also appeared that way for her before her piano pupil came into her world as well, so perhaps the same type of thing could be said for Watanabe?