The scariest outcome here is that a bunch of lunatics get ahold of a capable model and use to to harm the rest of us, who are at a disadvantage due to just how capable the model is.

But that's what's happening. The people in charge are a bunch of lunatics. However nice it would be to prevent them from having harmful capabilities, that ship has sailed. The best we can hope for now is preventing them from having supremacy, and that's what open weight models do.