[NotOscarWilde]

It is interesting to note two commonly repeated facts about academia, one of which you mention:

1. It is very difficult to get rid of aging professors with a low output.

2. It is fairly easy to get postdoc positions (but hard to get a permanent position).

In a way, you can argue that point 2 is a solution for point 1 -- with two postdoc positions (two years each, which is common at least in computer science) and a 4-5 year long PhD, you get 8-9 years of work from a candidate who is in his prime at the start and around say 30-32 years old at the end. After that, if you hired the candidate, you would only risk point 1, so why do that?

(I agree that it's incredibly mean to take 9 best years of a scientist while they work for a small wage, and then throw them away unless they are a genius. I think it's also important to keep in mind that while postdoc researchers face an extremely difficult challenge of getting a permanent position, they likely enjoy their academic work quite a bit. Postdoc/PhD are not exactly a "drain" in the same way a factory job might be.)