There is usually an independence war by everyone that isn't German within the first few years, followed by multiple attempts at lowering CA. I've played 100 years into an Italy game, trying to be friendly with the emperor and he hasn't even MENTIONED raising CA. France and HRE have dominated Iberia, Scotland won Greece from the Fatmids in a crusade (which they won like... 2 months into the game... Invasions are balanced guys) and I've united Italy. The HRE seems content just chilling with the default CA, partially because he has a ton of negative traits and partially because my character has negative relations with him while having a similar sized army.
I'm having trouble seeing what the problem is. The AI only very rarely rushes CA and when they do all their vassals revolt to replace him or lower CA. France literally falls to bits every time CA got raised, the guy gets replaced and CA gets reduced, back to square one.
Yes, thats your anecdote - there are others in the thread. So how to weight them?
Chain revolts are not news. But part of the problem is that it can be hit and miss. HRE usually can handle any revolt, and as a result the revolters are imprisoned indefinately . Independence revolts don't affect CA. And if the HRE dies early, or dies in battle, CA isn't affected (and if they, or the vassal dies, the malus for changing CA disappears). So in some games out of all this mess the HRE is abso and primo 50 years in, and in others its running between minimal and low.
One adds, a fair number of the HRE 'failures' aren't due to the CA process per se. The AI is prone to pick up amazing amounts of tyranny at times (-240 tyranny penalty!?) so you can look at a genius, grey eminence, long reign, positive trait emperor and find that he has every vassal at -100 (which is basicly 'pick your revolt faction'). Problem is thats another problem offsetting your first problem.