senile old people whose prognosis was grim

[suprunp]

Tengo watched this series of movements and wondered how she must feel to live her life in this sanatorium by the sea, taking care of senile old people whose prognosis was grim.
(H. Murakami; 1Q84)


Would you be so kind to explain to me why it is not written as follows: "...taking care of senile old people whose prognoses were grim."

Thanks.

[Raymott]

Tengo watched this series of movements and wondered how she must feel to live her life in this sanatorium by the sea, taking care of senile old people whose prognosis was grim.
(H. Murakami; 1Q84)


Would you be so kind to explain to me why it is not written as follows: "...taking care of senile old people whose prognoses were grim."

Thanks.

First "senile old" is a redundancy. 'Senile' means 'old'. At least it used to. Most people would probably claim that it means 'demented' now, because they are used to hearing it in the form of "senile dementia". But if senile means demented, then "senile dementia" is the redundancy.
To your question:
This is simply a way of saying "each of whom has a prognosis which is/was grim". Putting it this way, as a literary technique, makes all the people just one mass of senility with a grim prognosis.