Yeah, people seriously need to read the thread...
Changing to autocracy requires an autocratic faction to adopt into government. Getting said faction requires someone to support autocracy. Getting that someone may require boosting attraction towards autocracy (although you might be able to conquer those pops). And boosting the ideology without a faction requires slaves. And getting slaves requires autocracy. Egg and chicken, chicken and egg.
Well, almost. You can also get slaves through xenophobic ideals too, but that requires a more complex maneuver through ethics.
I think there should be more ways for autocratic attraction to rise - history hasn't really proven democracies to be immune towards becoming autocracies but Stella democracies sure seem to be so.