I'm not sure your strong criticism of Goodhart's Law is quite fair. It is stated so as to be pithy, but I think it is fairly obviously meant to be applied only to those situations where the thing being measured in some sense "judges itself" by that score, and adjusts its performance to try and improve that score.

This is quite likely the case with school children, but often not the case with, say, industrial processes. If, however, you make someone's job dependent on meeting some particular target for that process, then a feedback loop is created that can lead to the kinds of problems being discussed.

If the feedback loop isn't there, then the metric will remain as useful as it ever was. So the point is, be wary of unintentional feedbacks in your system, not some grandiose rejection of the possibility of optimisation.