Comment by shiroiushi

There's plenty of historical examples of monarchical governments where the king/queen/emperor was pretty good (or even great), but then died and was succeeded by their shitty son/daughter and things quickly went to hell.

The primary strength of a democratic republic (or a monarchy where the monarch has no power at all and is merely a figurehead, as is the case in many of today's monarchies) is avoiding this scenario. It doesn't guarantee stability, but monarchies are guaranteed to be unstable precisely because children are frequently not like their parents, and those systems don't have a way of removing shitty rulers, so you only get stability as long as the monarch is alive and of sound mind.