I dont get people who are complaining about the ending. I was hoping for something like that, and it was a nice surprise seeing the --- characters. Goutoge also answered what needed to be answered. As to why we dindt see grown-up version of the characters, I believe everyone forgets the manga took place in the '20s and that people that showed marks would die young anyeway. So, like, F.
And if that didnt kill them, world war 2 sure as hell would :vvv so I'm more happy with this "they married, they had children, they lived on" KNY wasnt a romance manga, it never was. But the characters who survived had their own happy endings,and these who died tragically recieved justice.
It wasnt a grandiose ending, but it was very satisfactory.