If your legal system depends on the benevolence of prosecutors, you've already lost before it began.

Attorneys are supposed to be adversarial. The system's soundness shouldn't depend on anything more than them trying to win and not doing anything illegal.

Before "prosecutor" became an elected/appointed office, prosecutors were independent contractors, hired for a single case only and serving at the pleasure of the Grand Jury. The Grand Jury's job was to decide how to spend the public prosecution budget. "Indictment" meant exactly that "prosecuting this person is a good use of tax dollars" and nothing more. We should go back to that.

Any system ultimately depends on the benevolence (or at least the decency) of the people in it. The idea that a society can design a perfect system and it will run itself is very dangerous.

The comment you're replying to was focused on prosecutorial incompetence, not benevolence.