The pyramid scheme for doing scientific research

Consider the following example (not precisely a real example, but close):

A University Department (not in Italy) has 20 tenured staff, 100 research staff (untenured) and 100 graduate students.
Suppose on average a tenured staff member remains in place for 30 years, and a graduate student takes 3 years to complete the degree.

Question: What proportion of students may hope to become tenured staff?

Answer: In 30 years 1000 students graduate, and 20 staff retire. So 1 in 50 students may hope to become tenured staff.

Do students do this calculation (admittedly rough) before commencing their graduate studies?

Do the tenured staff advise clearly students before they commence graduate studies?

Of course in some fields there is a need outside accademia for the skills and knowledge gained in completing a doctorate. But in the unnamed field of my example, though doctorates find work after their degree it almost invariably is quite unrelated to their doctoral studies. There is of course no objection to students doing a doctorate just for interest, while realizing that they must find occupation in a quite unrelated area.

How is it possible that 20 staff can manage so many students? That is what the 100 untenured staff are for. It is a pyramid scheme based on the naivity of students. Of course the untenured staff press the governments for more tenured positions but this annual growth rate is clearly unsustainable (a growth rate of at least 14% per annum at the most favouable interpretation).

I must admit myself to having supervised 8 doctoral students, well in excess of my quota, though in a period in which there was expansion of computer science jobs. Three have found permanent positions in academia and a fourth had the possibility - by choice only two of these are actually in academia now. Three found work in financial institutions, doing work which has nothing to do with their theses.

Exponential growth, with high growth rates, is clearly fundamental in situations like i) the initial growth of an organism or ii) the initial development of a new important, requested area iii) spring. But in the situation described above it is dangerous and dishonest. After spring there is winter, and then another spring.

I just noticed the following related cartoon.