I'm not sure I agree... lowering autonomy is entirely in your control as a player, right? Yes, regencies are chance-based and so is low legitimacy, but surely you could set this up in a way that you'd only be properly screwed if player actions had a role. I.e., if you annex your vassal and then never lower their autonomy and treat them nicely and live in peace, then a chance event alone shouldn't mess you up. But the way I understand the beauty of this disaster system, you wouldn't have to - because it would be a combination of things that pushes you over the line, some of which chance-based, some of which not.
Your provincial autonomy automatically lowers over time. Does this supposed 'lowering of autonomy' factor that in or is it just things the player actively has input in?