Tell that to Nicholas II of Russia. That's the mistake he made, as well as being a crap Czar. Similarly in WWI it was the constitutional monarchy in Britain and the republic in France that survived, not the autocratic states in Russia, Austria and Germany.
Or look at the history of China, Quite often dynasty's fade away over a long period of time but sometimes they don't last long. In which case its virtually always peasant unrest that is a big factor in their violent overthrow.
The treatment of the Huguenots were brutal and expanded over decades. However the point I think your missing was it occurred and lasted that long, with bad results for both them and France, because there was an autocratic monarchy would could get away with ignoring any warnings that "this is a bad idea sir". You see the same sort of thing with say Mao and Stalin as no one dare oppose them. With Hitler as well although his regime was so bad-shit insane that it only lasted just over a decade.
A corrupt democracy can act very similar to an oligarchy but it does have the chance of reform relatively easily, i.e. without the need for violent overthrowing of the current rulers. For instance the US has gotten rid of Trump despite many people being willing to break the law to maintain him. 30 years back Thatcher also feel when her irrationality was too much even for her party and the business interests that supported it, despite a terrible electoral system that enabled minority rule so long.