To his credit, intentional or not, according to his CV [1] he's only advised four PhD students and hasn't advised any since 2004. Four students per advisor lifetime might even be below replacement rate for having enough professors.
It’s important to point out the obvious: the replacement rate for academia is exactly 1 PhD student with faculty aspirations per lifetime of each professor. Anything >1 and academia must grow exponentially to accommodate every single PhD student who desires a faculty position. It is shocking how little this basic, obvious fact is discussed within our ivory towers.
Isn't it implied that only the best PhD students will get a faculty position? How do you choose the best one if you only let one student enter a PhD program?
This is particularly bad in academia because a lot of professors intentionally take on more PhD students than can possibly fill future roles for which a PhD is necessary, in order to use them as cheap labor. This is especially true in the biological sciences: rather than hiring career staff scientists to run experiments, professors simply use their much cheaper grad student labor pool.
What the exponent should be is a whole other matter.