At worst, a corrupt monarch could ... destroy the community, but the incentive do do damage to the community is roughly "for the lulz". Lulz is much cheaper elsewhere.

I admit, I have seen braindead moderators tear a community apart (/r/anarchism for one).

I have just as often seen lack of moderation prevent a community from becoming what it could. (4chan (though I'm unsure whether 4chan is glorious or a cesspool))

And I have seen strong moderation keep a community together.

The thing is, death by incompetent dictator is much more salient to our imaginations than death by slow entropy and september-effects. incompetent dictators have a face which makes us take it much more seriously than an unbiased assessment of the threats would warrant.