Also sometimes people don't actually know what's best for themselves.

For example, even a smart person with a PhD in neuroscience couldn't possibly understand the impact that government decisions will have on the economy, that's why we have people studying economics.

This goes for experts in every area, and often people shout "fuck the experts!" as they do the wrong thing.

We also have awful situations where an areas cultural or religious values fuck themselves over. Like middle eastern democracies who vote in madmen trying to initiate sharia law and end up causing massive conflict. It doesn't take a genius to see that decision is going to kill a lot of people but they don't place those values that highly.

What democracy is good at though (assuming it can be held together and isn't torn apart by corruption ) is making an already stable society progress towards a more ideal world incredibly incredibly slowly but reliably. A dictatorship or monarchy can send a country back to the stone ages if you have a shitty ruler and in comminism the need to progress in ideals is somewhat stifled by the fact that in pure communism is meant to already be perfect (it certainly isn't).

The only system not yet tried is a system where the best experts in the country rule but look at the house of Lords in the UK. They're meant to be an expert council but somehow smack-heads and dipshits have wondered in. So unfortunately if there's any hope that society is improving we're stuck with democracy as the only means to keep that happening.