Modern government gets replaced every few years, and each seems to suffer from 'not legislated here' behavior that makes them cancel slow-growth projects created by their predecessors (especially in times of financial crisis)
There are huge downsides to monarchies and dictatorships, but that's one of the few upsides that I haven't been able to see in Democratic systems.
With huge problems facing our civilization regarding our ecosystem and climate, the solutions might require multi-generational solutions.
Also, does anyone else think that a democracy might not work on a multi-generational interstellar ship?