But I found I agreed with the main points:
1) There's little chance your frank feedback will have a positive impact
2) There's small but real chance of a downside you can't fully quantify. Even this possibility may cause you stress.
What I think is missing is that before you decide to leave, do give honest feedback. I don't think shying away from conflict for fear of retaliation is a good idea - you can shape the place you are still in. But once you are gone, I think the chances are smaller, and you have already decided to left in any case, which probably means you believe whatever you are leaving about will not change.
If you have been giving honest feedback all along, and then do decide to leave, the people you trust will already know why you are leaving and can choose to take the signal that you felt strongly enough to leave to heart if they chose.