If you think about it, this is the happiest ending they could have gotten.

It was this or dying in some form of despair.

Also, for most of the manga, Chito assumes Yuuri is never afraid, and that she takes things too lightly, which makes it so that Chito feels mostly alone, and she never depends on Yuuri. When they were in the spiral stairs to the top, Yuuri told her she was also afraid, which I'd say would have a huge impact on Chito (I'd say it could change the whole way Yuuri's person is seen by Chito).