You seem to be under the misconception that the purpose of democracy is to select the objectively best governmental policy, as if such a thing could ever exist. That is not the purpose of democracy. If you want that kind of system, you want technocracy, not democracy.

The purpose of democracy is to keep us from killing each other. THAT is the purpose of democracy. It's too give enough people enough of a voice in government that very few people think starting a civil war is a necessary and proper course of action. It has nothing to do with selecting the "best" governmental policy, as that's entirely subjective.

The reason you don't want filter voters based on intelligence is that stupid people can use firearms just fine.