Nice to see that you guys are trying to address the issue
I would argue that the point of most players quitting when a civil war happens is irrelevant. The challenge of the player is to
avoid the civil war. If it happens (notice I said "if" rather than "when"), then they can choose to persevere or quit.
The main problem with catastrophic events in games is that the player is often given little warning and/or can't prevent it from happening. The more catastrophic an event, the rarer it should be, the more warning the game should provide, and the better the ability the player has to prevent or alleviate it.
Consider the American Civil War. It did not just happen. There were plenty of events leading up to it. A significant enough portion of the population needed to be unhappy, such that the new nation would have a chance at winning the civil war. The succession also did not necessitate a civil war. In the cae of the the Civil War, the Union started it - it could've been a peaceful split if the Union had a different Congress and President. Futhermore, the unhappy population needs to have the means to organize and overthrow anti-succession elements within their regions. For example, unhappy populations are less likely to rebel if the government is oppressive (of course, oppressive govts make populations unhappy, but that's another story). The moral of the story is that there should be plenty of opportunities to stop or alleviate a catastrophic event.
In any case, catastrophes should be the minority of events. Some events will be big, most will be small. Some are bad, some are good. Most events should be things on the scale of "we love the monarch day" stuff. I'd consider a quest system to be a type of event generator as well.
Another thing is that events should be happening to all other players as well, regardless of whether they are AIs or not. It just makes the game a lot more interesting. Imagine a civil war splitting an orc empire into two. Or a good civ change into an evil one. Or a drought striking an enemy. Point is, events don't just have to happen for the human player; it can occur for others yet still make a large impact on the game.
EDIT: Forgot to mention that I forgot the difficulty of the game I'm playing. It's on some medium-low difficulty. As I expected, it didn't take long for me to get to the top, but I just couldn't envision a case where once I'm at the top, I could ever be defeated, regardless of difficulty.