Even if we had known, you can argue that the charge that moves around in circuits “should” be the positive one, or you can argue that the charge that stays put “should” be the positive one. Franklin didn’t “get it wrong”, because there is no “right”.
Likewise with all of the various right-hand-rules. Students are often taken aback when they learn that all coordinate systems must necessarily involve an arbitrary choice that can’t be mathematically defined.