The Mide campaign Kingdom Victory requirements currently demand the player to annex four factions. This is done by acquiring enough of the Legitimacy currency (20) and spending it. However, the amount of Legitimacy the player is able to earn is disproportionate compared to the amount lost to "Natural Decline". I am simply not able to gather enough Legitimacy, and If I want to gather enough to use for annexation I will never be able to use it to enact edicts.
Yes, I could earn some more legitimacy by conquering all the factions I want to annex, but that is a bit counter-productive.
I have not been able to come up with a reason why removing natural decline would cause any imbalance.