This sort of pathology is not present on all court systems. In many countries the prosecutor is not judged on "performance" metrics like conviction rate but on merit as judged by his senior peers. Objectively the job of the prosecutor is not to convict the maximum number of people but rather to establish the truth and help the judge impose proportional sentences based on facts.

It's not a foolproof system but it seems to work much better than ones with "objective" criteria