very interesting study subject. I just have one question or comment that immediately came to my mind.
One of your conlusions is (advice for funders and, maybe, PIs) that it might be better to invest in permanent staff than in post-docs or Ph.D. students. Although it would often be nice to have more funding for permanent staff, there may be other considerations: It is no news to most PIs that training Ph.D. students is a complex task that cannot solely be measured in terms of immediate productivity. There we go again, in the direction that has been discussed numerous times in a broad context already: should impact factors be the only measure that contribute to quality measurement for the funding and future of a scientific/academic group, or are other factors, for instance the training investment into Ph.D. students and other junior group members a value in itself that should be appropriately rewarded and remunerated by specific "factors" that count towards a specific academic success factor and towards success in scoring funding? I agree that changing the system is difficult seen that scientific success is currently measured as number and "impact" of publications without looking at the status of the authors.
You state (citation) that " most post-docs never obtain a permanent position, which means that the
training they received is wasted, at least within academia." This may appear like a a quite simplistic view. Many ex Ph.Ds or Post-Docs obtain very good positions outside of academia, found companies, work in regulatory offices or the government, maintain contacts and are valued and important members of the society.
On the other hand, and this has always been a little backdraw to the science system, that before starting the training, you don't know how many, who, and for which reasons will stay in academia in the end. There is no way to know that, since it is a multifactorial decision. However, if no Ph.D.s or Post-docs are trained, nobody will make it in related fields in or outside academia. What can be the consequences?
Most of us PIs have been Ph.D. students at some point and have benefited from the many training hours invested in us. For many group leaders, it is also an absolutely essential thought to give back what has been invested in us a while ago to the young scientists-to-be and to invest in their training. This system of propagation is also essential to the future and survival of science, and there is no other system to be seen anywhere in science replacing the system of close mentoring.
Maybe it is worthwhile to mention some of these musings in your discussion.