[quote=Yog Sothoth]
Still, remember star wars? The sith took power because they sold the deception of an enemy to the galactic republic, so just because it's a UN type of republic doesn't mean that evil schemes can't be hatched from within it
[/quote]

The difference between the star wars republic and the Shinobi republic is that the Shinobi republic doesn't act as a governing body over everything - each shinobi village still has control over their respective armies, so to speak, meaning they have to keep each other in check. The Star Wars Senate was, quite literally, one single nation. In truth, there is very little similarities between the two republics. 

The only way I see this changing is if the republic shinobi nations all agreed to become one nation, so to speak, and choose one supreme leader, or supreme Kage. But until that happens, the critical fact each village is capable of thinking for and leading itself adds credibility.

EDIT: Or I might just have misread some of the backstory if any of that turns out to be wrong.