>> Their advantage over dictatorships or other forms of government is merely that they "effectively prevent lower-than-average candidates from becoming leaders."

Of course they don't elect the "best" leader. What is that, anyway?

Democracies exist only to have an obviously legitimate system to replace the current leaders by new ones every few years, and thus avoid corruption from growing too large and damage the society/economy.

The difference between the "best" or an "average" leader is much less than the difference between "little" and "a lot of" corruption.