I dunno, it was sort of the opposite for me. I saw it as a sign that they loved long and well, and came to the end of her life not bereaved, but fulfilled. Something like MarAli, where Marisa passes well before Alice but while there are tears it's not necessarily from sadness.
Oh i sobbed reading MariAli. In both cases it was "happy tears", I'm an emotional creature so when I'm angry; I cry, when I'm sad; I cry, etc.
I agree completely with you, Keine passed after hundreds (?) if years, and yet she left a Mokou who has been healed and has had many many days of happy memories. I'm not sure if it was supposed to be read this way, but I saw the happy Mokou, with a bright smile saying "a lot", as the one answering the old!Keine's question.