Those professors need skilled people to actually do the work. As soon as postdocs aren't simple low-wage migrant labor, their usefulness is reduced.
While this makes sense in some areas of research, my impression is there are areas of research where there is very little work that can be relegated to the postdocs. I've found in mathematics for instance, it isn't at all uncommon to see a postdoc doing something nobody else in the department is doing.
I imagine there are at least some areas of theoretical physics where the same sort of remark holds.
Yet, why is the system roughly the same in that case? I don't see the benefit of having so many more postdocs than tenure-track (that is, people with a reasonable chance of remaining at the university given that their research is strong).
Why would you expect a new graduate to be better trained than a postdoc that has recently completed a research project in the field?
Exactly, I agree with this. I'm not sure if I'm missing something, but I see almost no reason not to believe this.
there is no practical evidence to suggest that doing so will have any major benefit, and (c) even the theory that it would be helpful is debatable.
If expendable labor that full-time faculty can relegate to those of lower rank is the goal, then this is true.
But in the cases where little such relegation of labor is practical, I'd presume the obvious goal to strive for is to get the most and best research output possible for the amount you pay your researchers. Perhaps it is true that those who obtain tenure at, say, MIT are so absurdly above the leagues of most researchers that there's no reason to contemplate hiring too many others.
I have heard of systems where tenure-track positions are effectively not tenure-track, since nearly nobody gets tenure, but I only hear of these much at universities like Princeton. I wonder if a better model than lots of expendable postdocs and a reasonable number of tenured faculty is to have more in the middle, and fewer at top. I think it would encourage more of the bright postdocs with good ideas to stick it out and produce lots of things. Maybe they won't get tenure, but if they aren't sent away in favor of a newbie every few years, maybe they'll stick it out longer and produce things they really couldn't have as newbies (or, for that matter, disgruntled people who walked away from academia to a different career).
After all, universities pour a lot of money into graduate students (paying tuition + for TA duties), and they offer tenured faculty significant benefits, so I can't help but wonder why there's less in between, since I've heard time and again that the in between phase produces a lot of the best research (in between when you're too old and too young to do anything impressive).