Even a relatively regional event can profoundly damage a nation's economic balance. Look at the historic Ming, look at the french revolution, there is really no lack of evidence. After all, ming's "mandate of heaven lost" modifier is just a stylized event that could happen to each and every country.
Introducing a trade-off is the opposite of "getting rid of player choice and agency".
Vassals and colonial nations evidently should not count towards your stab costs. There was a reason the british empire organised it self the way it did (" The British Empire comprised dominions, colonies, protectorates, mandates and other territories. Dominions were semi-independent polities that were under the British Crown, constituting the British Empire, beginning with Canadian Confederation in 1867. They included Canada, Australia, New Zealand, Newfoundland, South Africa, and the Irish Free State, and then from the late 1940s also India, Pakistan, and Ceylon.").